$15.95

Preface

Wireless systems are a key component in almost every broadcast, motion picture, theatrical and sound stage production, as well as corporate, religious and educational venues. As the airwaves continue to become more congested with a myriad of devices and the demand for more systems increases, the need to understand the concepts behind the design and operation of wireless systems becomes a critical concern for professional users. The emergence of DTV (digital ) broadcasts in North America has created an increasingly difficult environment for operating wireless systems as the available spectrum becomes more congested. DTV is also emerging in Europe, as another indicator of the spectrum crowding that the future holds in store. Considering these facts, and the ever increasing popularity of wireless , intercoms, IFB systems and other communications devices used in almost every production, the need for a solid, technical understanding of wireless systems is at an all-time high. This guide is intended to uncover some of the mysteries behind the operating principles of wireless systems and help the reader separate fact from fiction when selecting a wireless system for a particular application. Far too much propaganda has been published by various manufacturers, with boastful, and sometimes incredible claims about the quality of the equipment being offered. Armed with an understanding of the basic principles of wireless operation, it becomes possible to see through the maze and make intelligent choices. Our philosophy at Lectrosonics is to build the best product we know how to make, and support it with the best service possible. This includes the publication of guides like this one, and maintaining a highly responsive attitude toward the market. Please feel free to contact us regarding any of the material presented in this guide. Your comments, suggestions and experience are extremely valuable to us.

Bruce C. Jones Vice-president, Marketing Lectrosonics, Inc. March 2000

Copyright © 2000 by Lectrosonics, Inc.

Contents

Page

BASIC WIRELESS SYSTEM CONFIGURATION ...... 1

TRANSMITTERS...... 3

RECEIVERS ...... 7

DIVERSITY RECEPTION...... 13

AUDIO ...... 19

ANTENNAS ...... 23

FREQUENCY SYNTHESIS ...... 27

INTERFERENCE ...... 31

FREQUENCY COORDINATION...... 37

MULTI-CHANNEL WIRELESS SYSTEMS and MULTI-COUPLERS ...... 41

SIGNAL TO NOISE RATIO...... 45

INTERPRETING WIRELESS MIC SPECIFICATIONS...... 47

EVALUATING WIRELESS MICROPHONE SYSTEMS ...... 51

GLOSSARY OF WIRELESS MICROPHONE TERMS...... 55

WIRELESS MICROPHONE APPLICATIONS ...... 57 Basic Wireless System Configuration BASIC WIRELESS SYSTEM COMPONENTS

A wireless microphone system is a highly specialized combination of RF () and audio electronics that replaces the cable normally used to connect a microphone to another audio component. There are basically three components in a wireless microphone system: Belt-pack 1) Microphone 2) Transmitter 3) Receiver The term “system” refers to a combination of these three components.

The microphone in a wireless system may be an integral part of the transmitter Plug-on transmitter or it can be a separate component. In many cases, the microphone is a separate component purchased apart from the wireless system. A huge variety of differ- ent types of microphones are available to meet any application. The application normally dictates the type of receiver needed, and the microphone requirement normally dictates the transmitter model needed.

Hand-held transmitter

Wireless are available in three different configurations for various applications: • Belt-pack models • Hand-held transmitter/microphone combinations • Plug-on models

Wireless receivers are also available in a variety of configurations for various applications: • Compact models for field production • Table-top models for sound reinforcement • Rack-mount models for studio and stage Compact receiver

Table-top receiver Rack-mount receiver

1 Wireless Microphone Systems

The total number of wireless systems needed in a single SPECTRUM USAGE AND FREQUENCIES location often points to the need for specialized RF distribu- The vast majority of high quality wireless microphone tion, antennas, cabling, receiver mounting assemblies and systems use a method of radio transmission called “FM” other accessories such as the following: (Frequency ). In FM systems, the radio signal (the • multi-couplers carrier) is frequency modulated (the frequency is increased • Quad-pak receiver systems and decreased) by the audio coming from the microphone. Another method of radio transmission, “AM,” (Amplitude • Remote antennas and cabling Modulation) is commonly used for communications and • In-line RF filter/amplifiers and splitter/combiners voice-band applications. In general, FM produces better audio than AM, so the FM principle is used almost exclu- sively for wireless microphones. In the USA, wireless microphones operate in frequency bands Rack mount multi-coupler specified by the FCC (Federal Communications Commission). Bands have been allocated for wireless microphones in the VHF spectrum from 150 to 216 MHz, and in the UHF spectrum from 470 to 806 MHz. These bands are used almost exclusively for television broadcast, except for a small portion of the VHF band between 169 and 172 MHz. DTV () broadcasts are being allocated in the UHF spec- trum in what were formerly empty channels. The upper and Quad Pak multi-coupler lower parts of the UHF spectrum are also being broken up and re-allocated to make room for additional services. As the available spectrum space for wireless microphones continues to shrink, the need for higher quality wireless microphone systems increases dramatically. High powered television broadcast signals on adjacent channels can render a previously working wireless system virtually useless. High performance receivers and multi- couplers, and specialized antennas and cabling equipment are required for any professional application to meet the con- stantly increasing demand for more and more wireless microphone systems. As you can well imagine in this “digital” world, extensive engineering effort is also being applied to develop digital modulation techniques for wireless microphone applications. The hopes are that digital wireless systems might alleviate some of the problems encountered with the present analog FM systems that now exist, and produce equal or higher Folding quality audio performance. It is much too early (as of the date of this writing) to delve into the intricacies of compara- tive techniques, so this guide is focused on the FM principle. Present digital technology has produced good quality cellular systems, but the limited audio of even the highest quality telephone system cannot produce the audio quality needed for even minimal LPDA antenna wireless microphone applications. (Log Periodic Dipole Array)

In-line RF filter/amplifier Passive 4-way RF splitter/combiner

2 Transmitters

TRANSMITTERS OVERALL CONFIGURATIONS Transmitter designs can be grouped into three basic types: Plug-on “Belt-pack” lavalier models for use with lapel mics, instruments, mixers, tape decks, etc. “Plug-on” models for use with hand-held mics, boom poles, mixers and other equipment with XLR connectors. “Hand-held” models with an integral microphone capsule, primarily for hand-held use.

Belt-pack

“Plug-on” transmitters adapt any microphone with an XLR connector to wireless operation. Their usefulness is evidenced by the increasing number of models offered by various manufac- turers. Professional models typically operate on alkaline or lithium 9 Volt batteries, and provide a wide input adjustment range to accommodate the wide variety of microphone types. The mechanical construction, especially the input coupler, is the key to what makes this type of transmitter most useful for Belt-pack transmitters can be constructed of metal or professional applicaitons. plastic. They can be designed to utilize the micro- phone cord or input cable as the antenna (common in VHF designs), or use a “whip” antenna. Input gain must be adjustable over a wide range to accurately match the enormous variations in output levels from Hand-held microphones and other equipment. Most belt-pack transmitters operate on alkaline or lithium 9 Volt batteries, and include a battery status indicator of some type. The best designs offer adequate warning of low battery condition to allow time to plan for a battery change. The battery door design is also an important consideration. If the battery door detaches from the transmitter, it can get lost or broken, rendering the system unusable. The battery compartment must accept the wide variety of different brands of batteries available. The belt clip is oftentimes a weak point in many belt-pack designs. It should provide a secure attachment, but also be easily removable for conceal- Hand-held transmitters with integral microphone capsules are offered by ment in applications such as in motion picture or almost every wireless mic manufacturer. The most popular applications for stage productions. hand-held transmitters are in the musical performing arts. A hand-held transmitter must be comfortable and secure in the user’s hand to guard against being dropped. It must provide the necessary level and frequency adjustment controls, yet the controls must be concealed or recessed to prevent accidental mis-adjustment during normal handling. Frequency selection controls are provided in various manners in synthesized transmitters, ranging from pushbuttons with LCD readouts to concealed rotary controls. The example shown here provides two rotary switches to select one of 256 different frequencies over a 25.6 MHz bandwidth. The left-hand switch changes the frequency in 1.6 MHz steps and the right-hand switch provides 100 kHz steps. Each switch has 16 positions, providing a total of 256 different frequencies.

3 Wireless Microphone Systems

USER CONTROLS AND INDICATORS With regard to all types of transmitters, the user controls and it can be visually distracting and vulnerable to breakage. Hand- indicators are the practical features on various transmitter models held transmitters designed with internal antennas eliminate this which can “make or break” a design as it serves a specific vulnerability and are generally the most visually acceptable. application. For example, if a transmitter is to be used by a Plug-on transmitters use the metal housing of the transmitter variety of different users, it is imperative that it offer an accurate body as the antenna, with the attached microphone and even the means of compensating for the different voice levels of the user’s hand forming the other half of an ideal “dipole” arrange- various users. Proper gain adjustment is critical, since this will ment. At UHF operating frequencies, the length of the housing is determine the ultimate signal to noise ratio of the system. very close to an ideal 1/4 wavelength, which provides maximum Without a visual means of monitoring the audio level, it is almost radiated power and exceptional operating range. impossible to correctly adjust the input gain to the transmitter. Although this adjustment can be made by viewing level meters INPUT GAIN ADJUSTMENT on the receiver, it is much more practical to monitor the levels at This is an area where wide variations will be found in transmitter the transmitter since, in many applications, the receiver is not designs from different manufacturers. Simply stated, setting the within view from the transmitter location. proper input gain is the most important adjustment on a wireless A common problem in some designs relates to function switches microphone system, yet it is often overlooked in many designs. that are too easily moved. This can create numerous problems, Set too low, the signal to noise ratio of the system will suffer. Set including the complete “shut down” of the system during a too high, severe distortion and/or compression of the dynamic performance. The location and physical adjustment of opera- range will occur. Adjusting the transmitter input gain is very tional switches will determine the usefulness of a transmitter for much like setting the record level on an analog tape recorder. a specific use. For example, a public speaker may require that It is important to consider the features and controls offered on the transmitter be muted easily, even when the transmitter is any wireless transmitter that enable accurate gain adjustment. worn underneath clothing. In this case, a switch that is hard to LEDs on the transmitter can be used as well as some sort of reach can make it difficult to operate. In many stage productions metering on the receiver to adjust the input gain. It is best to the sound company may require that the user has no means of have indicators on both transmitter and receiver, however, so changing any switch or control on the transmitter. These two accurate level monitoring is possible from either the transmitter examples are mutually exclusive. or receiver location to accommodate a variety of applications. Lectrosonics hand-held transmitters offer internal switches that defeat the external switches, which allows the transmitter to be configured for any application. ANTENNA CONFIGURATIONS The antenna configuration is another consideration in comparing different transmitter designs. If every user of a wireless system could live with mounting a metallic “tree” on their shoulder, or on top of their head, transmitters could radiate RF energy very Typical efficiently, providing incredible operating range and few (if any) transmitter drop outs. Few people, however, would wear something like that gain control Receiver front panel LED in public. So, in reality, the transmitter antenna must be inoffen- with LEDs for strip for monitoring. sive or invisible altogether, yet it must still radiate sufficient RF adjustment. energy for the receiver to operate normally. The only reason that wireless mic systems work as well as they do is the fact that the operating ranges are generally fairly short (several hundred feet, OUTPUT POWER or less) and adequate RF power output and modulation (devia- tion) is allowed by the FCC to let the systems operate with an The maximum allowable RF output power produced by the final acceptable signal to noise ratio. amplifier in the transmiter is regulated by the FCC. For example, in the VHF spectrum from 174 to 216 MHz the maximum A common problem with belt-pack transmitter antennas is that allowable transmitter output power is 50 mW. In the UHF band they are normally worn against the body, especially when they maximum allowable transmitter power is 250 mW. are concealed in costumes. By placing the antenna in contact with the user’s body, much of the radiated RF energy can be lost Higher output power from the transmitter helps overcome drop before it can get out into the air, which reduces the range of the out problems and increases operating range, but the sacrifice is wireless system. When used with portable mixers in over shorter battery life. The actual effective radiated power is heavily shoulder bags for field production, the transmitter can be affected by the individual transmitter antenna, so a higher output positioned away from the body and other equipment to allow the power does not necessarily mean greater operating range. maximum radiated power and operating range. Most high quality VHF transmitters produce the allowed 50 mW, In the case of hand-held transmitters, the unit is normally held for reliable operating range and reasonable battery life. There out away from the users body, with the user’s hand actually are some VHF designs offered by some manufacturers, however, becoming part of the antenna. A protruding “rod” antenna that produce only 30 mW or less, yet the published specifications moves the antenna out of direct contact with the user’s hand, but for these models make it appear that they are full power designs.

4 Transmitters

UHF transmitter output power varies much more widely from one brand to another than VHF units. The maximum allowance of 250 mW in the UHF spectrum is useful when maximum operating range is the prime concern. The trade-off is shorter battery life. 100mW is commonly chosen for UHF transmitters as a good balance between operating range and battery life. Some UHF models are advertised to radiate as much as 150 mW when, in reality, they actually only produce 30 or 40 mW. There is no excuse for any manufacturer to publish incorrect informa- tion about their product simply to disguise the fact that it may not meet the competition, or may have some inherent design problems. It is also interesting to note that these same manufac- turers often forget to publish battery life or power consumption in their specifications. POWER SUPPLIES AND BATTERY LIFE Battery life is a common concern in considering the use of any transmitter for a particular application. It is a critical concern in applications such as motion picture production and theatre where the transmitter is often concealed underneath elaborate costumes and changing a battery could be a major ordeal. An accurate transmitter battery status indicator should be provided on the transmitter and on the receiver for this type of application. A 9 Volt alkaline battery (the most commonly used for wireless mic transmitters) starts off slightly above 9 Volts and then drops down gradually over the life of the battery. High quality transmitters include internal voltage regulators to keep the transmitter stable as the battery voltage drops. Extended operating time can be achieved without sacrificing output power with a design that allows the circuitry to continue to operate at lower battery voltage levels. The best designs continue to operate at battery levels down to about 6.5 Volts. High-end wireless systems normally provide a tranmitter battery status indicator on the receiver to warn the user well before the transmitter ceases to operate.

5 Wireless Microphone Systems

6 Receivers

RECEIVERS OVERALL CONFIFURATIONS The application for a wireless microphone system dictates the The process sounds simple when described like this, but the type of receiver required. In general, receivers can be sorted into reality is that designing a truly high quality FM receiver is a bit a few broad categories: of a “black art.” Super-het receivers can be “single conversion,” • Compact models for field production on cameras, in “double conversion” or even “triple conversion.” The diagrams portable bags or on sound carts; also includes multi-channel below depict highly simplified single and double conversion FM assemblies for motion picture production receivers. A triple conversion design simply has a third oscillator and mixer. The drawings depict only the major stages in each of • Table top models generally used for “stand alone” applica- these receiver types for the purposes of this discussion. In reality, tions in sound reinforcement each of these stages usually consists of a number of separate • Rack mount models for high-end studio, stage and mobile circuits and sub-circuits, some providing a basic function, with production vehicles others providing additional correction or control. As you can The differences between these basic groups have to do with the well imagine, each stage in the receiver provides a challenge to physical configurations, powering options, RF performance and the design engineer to meet the performance and cost criteria of audio performance. Within each of these groups, a wide variety the design. The mechanical design of good receiver must also be of different models are offered by dozens of manufacturers, over integrated with the electrical aspects, in order to provide the a very broad price range. necessary shielding around some sections of the circuitry. FIRST A FEW BASICS RECEIVER FRONT-END DESIGN In order to make comparisons between various different receiver The front-end of the receiver is the first step in a chain of designs, it is important to have at least a basic understanding of filtering, gain and conversion processes. The front-end is receiver design. With a basic understanding of the various basically a bandpass filter operating at the carrier frequency of sections of a receiver in mind, the differences in cost and the wireless system. The job of the front-end is to reject high performance offered in two different models will become more powered RF signals above and below the operating channel and clear. This will help significantly in evaluating systems for to provide strong “image rejection.” (“image rejection” is specific needs and making valid purchasing decisions. explained later in this chapter) The front-end can consist of either simple low cost coils to provide simple filtering for a low FM receivers for wireless microphone systems are super-het cost design, or it can consist of helical resonators or tunable (super heterodyne) designs. The “super-het” process involves ceramic resonators for high performance designs. generating a high frequency signal inside the receiver and mixing or “heterodyning” the signal with the incoming carrier signal. A simple coils in the front-end section provide only broad band When the signals are mixed together, the intermodulation filtering, which is oftentimes not enough to guard against produces “sum” and “difference” signals. The purpose of mixing interference from high powered RF signals close to the operating the signals is to derive a lower frequency signal which can then frequency of the wireless system. Television broadcasts are the be processed with conventional circuitry. Through filtering, the most common high powered source of interference for UHF “sum” is rejected, passing only the resulting “difference” signal wireless microphone systems. As DTV (digital television (the “IF” or “intermediate frequency” signal). The IF signal is broadcasts) fills up formerly empty spectrum space, the need for converted to audio in the detector stage, followed by some type high quality, narrower front-end sections increases. Helical and of audio output amplifier. Thus, a radio signal is converted into tunable ceramic resonators, used in multiple stages with high an audio signal. quality amplifiers, are the best means of reducing or eliminating interference from television broadcast signals.

RF FRONT IF IF END MIXER FILTERS IF AMP DETECTOR FREQ

AUDIO AUDIO EXPANDER OSC AMP

1ST 2ND IF SIMPLE IF MAIN MIXER FILTER MIXER FILTERS IF AMP DETECTOR RF FRONT END AUDIO 1ST 2ND EXPANDER AUDIO OSC OSC AMP

7 Wireless Microphone Systems

COMPARATIVE FRONT-END FILTER SLOPES signals inside the receiver. Two signals are injected into the receiver to produce third order IM on the carrier frequency of the CARRIER receiver and the level of the IM product is measured. Through 0 dB LOW Q several different measurement techniques, an accurate calculation -20 can be made as to the input level required to produce this effect. The rated third order intercept number is an excellent means of -40 measuring the receiver’s IM rejection. -60 The type of amplifier used has a large effect on the third order -80 intercept performance of the receiver. Amplifiers with excellent HIGH Q third order intercept performance require lots of power, which -100 poses a problem with a receiver designed for battery operation. The narrower the front-end filters (which cost more), the less apt FREQUENCY MHz the receiver is to pick up signals that could generate IM. The difference in performance between various front-end designs IMAGE REJECTION is primarily two areas: Image rejection is a major performance measurement of a • Selectivity receiver. There are two signal frequencies which will combine • IM (intermodulation) Rejection with the local oscillator to produce the same IF. One is the desired signal from the transmitter and the other is the frequency Selectivity is expressed by the amount of signal rejection above which is the same “distance” from the local oscillator as the and below the operating channel the front-end will provide. The desired signal, but in the opposite direction. RF energy on or comparative curves in the drawing below illustrate the slopes of near the image frequency of a receiver can be a major source of various types of front-end filters. The steeper the filter slopes, interference. the stronger the rejection of energy on adjacent frequencies. Different types of front-end components (coils, resonators, etc.) IMAGE REJECTION vs. FRONT-END SLOPE produce varying filter slopes, but all high quality receivers are IMAGE OSCILLATOR CARRIER designed with multiple stages of whatever components are used 0 dB in the design. These multiple stages increase the filter slope -20 dramatically, but do cost more. -40 Intermodulation is the mixing of signals to produce new signals. For example, when two signals are mixed into an active circuit -60 such as an amplifer, the output of the amplifier will include both -80 signals, plus the sum and difference of the signals. The sum and -100 difference signals are called “IM products.” Third order IM simply means that the second harmonic (second order) of one of IF FREQ IF FREQ the original signals (Fa) mixes with the fundamental (first order) It is not unusual that the image frequency of a wireless micro- of the other signal (Fb) to produce another signal (Fc). phone system operating on an empty TV channel could be at the Or: 2(Fa) - Fb = Fc same frequency as another signal. This will Preventing third order IM is most important because it produces produce interference problems for all but the most selective strong interference products and the two signals that produce the front-end designs. Sharp front-end filtering rejects energy on the third order IM can be arbitrarily close to the desired carrier. If image frequency to keep it from entering the receiver. the interfering signals do happen to be close to the carrier frequency, then the front end filtering is largely ineffective. The CAPTURE EFFECT only way to reject this kind of close in IM interference is to have FM receivers benefit from an high overload point amplifiers and mixers in the receiver. effect called “FM capture.” This For example: refers to the fact that an FM receiver will capture more audio Given two frequencies of 645 and 650 MHz from a strong signal than from a then weaker one. The audio present 645 MHz x 2 = 1290 MHz in the stronger signal will be and dominant in the audio produced 1290 MHz - 650 MHz = 640 MHz at the receiver output. The A receiver at 640 MHz will pick up direct interference from two weaker signal, however, can still tranmitters on 645 and 650 MHz. Even spacing like these three increase the background noise frequencies is always taboo in wireless microphone systems. and make dropouts occur more often. A “weaker signal” in this IM performance is rated with a specification called “third order sense could be another wireless intercept.” This spec is a number expressed in dBm that refers to transmitter signal, or broadband the power of interfering signals needed to cause the receiver to background noise. produce distortion (IM) at the same level as the interfering

8 Receivers

RF MIXERS The mixer in a receiver combines the incoming RF signal with the oscillator signal, producing “sum” and “difference” signals. The “difference” signal is at the desired IF frequency. Low cost 0 RF mixers produce spurious signals (harmonics) along with the -10 desired sum and difference signals. If spurious signals occur -20 near the IF frequency of the receiver, they cannot always be rejected by the IF filters and can cause noise or distortion in the -30 final audio output. High quality RF mixers produce only a sum -40 and a difference signal, without harmonics. The “sum” is so high -50 4-POLE CERAMIC in frequency that it is completely rejected by the IF filters -60 IF FILTER following the mixer, leaving only the desired “difference” signal for subsequent processing. -70 -80

dB SUM: 370.700 10.310.4 10.5 10.6 10.7 10.8 10.9 11.0 11.1 MHz IF FILTERS & 190.700 MIXER @ 10.7MHz DIFF: 10.700 10.7MHz IF 0

180.000 -10

OSC -20 -30 6-POLE CRYSTAL The mixer must also have a very high overload threshold. -40 IF FILTER Overload can occur when the total RF energy being fed into the -50 mixer exceeds its capacity. Sharp front-end filtering reduces this -60 problem, but strong signals a few MHz away from the carrier frequency can still get through the front-end and cause overload -70 in the mixer. The most effective approach in front-end design is -80 to include only enough gain between each filtering stage to dB compensate for the required losses. The basic idea is to apply all 21.021.1 21.2 21.3 21.4 21.5 21.6 21.7 21.8 MHz the required filtering before any significant gain is applied to the signal to keep noise and interfering signals at a minimum. 0

IF FILTERING -10 The IF filtering in a receiver produces the real “selectivity” in the -20 receiver. Standard multi-pole ceramic IF filters offer a bandwidth -30 SAW of about 300 kHz. Six-pole crystal IF filters offer a bandwidth FILTER only 45 to 50 kHz wide. The narrower the filtering in the IF -40 stage, the better. Crystal filters cost many times more than -50 ceramic filters, but the performance is worth it in many applica- -60 tions where interference is a serious problem. -70

Narrow-band crystal filtering, however, demands that the -80 frequency cannot drift, which requires a very temperature stable dB oscillator. Low cost, “wide-band” receivers can get away with 21.121.2 21.3 21.4 21.5 21.6 21.7 MHz “drifty” oscillators, since the resulting IF frequency will still be within the limits of the IF filters. A third filter type that is beginning to see wider use in receivers Without question, the best interference rejection occurs with a is the SAW filter (Surface Acoustic Wave). These filters use stable oscillator and narrow-band IF filtering, as is produced with surface waves on a quartz or other piezoelectric material to crystal filters. The only drawback to crystal filters, however, is transfer RF energy from input to output, and use the precise they have a slightly higher distortion than ceramic and SAW spacing of interdigited fingers on the surface to pass some filters when the signal is highly modulated. For this reason, you frequencies and filter out all others. SAW filters offer precise will often find ceramic or SAW filters rather than crystal filters filtering at higher than usual IF frequencies and also provide used in high-end receivers where the primary concern is audio minimal phase shift (group delay) that is difficult to achieve with fidelity. other methods. This makes the designer’s job a bit easier as wireless microphones use higher and higher UHF frequencies, but while they have higher selectivity, they are more expensive than some other types of filters.

9 Wireless Microphone Systems

FM DETECTORS STABILITY AND THERMAL DRIFT The discriminator or detector in a receiver is the circuitry which A second benefit of a double inductors and capacitors) in a converts the frequency modulated (FM) radio signal into an quadrature detector circuit vary for any reason (usually tempera- audio signal. There are several different circuits used by various ture), serious distortion will be produced in the audio signal. For manufacturers, but all detectors in wireless microphone receivers example, in a single conversion receiver operating with an IF at are based upon two basic approaches: 10.7 MHz, a drift of only 0.5% in the tuning of the detector 1) Quadrature Detector circuit would result in the detector being over 53 kHz off frequency. This is enough to cause serious distortion. In a 2) Pulse Counting Detector double conversion receiver with a second IF at 1 MHz, the same A quadrature detector is a circuit that utilizes phase shift to shift of only 0.5% in the detector will result in only a 5 kHz shift generate a varying DC voltage, producing the audio signal. The in the tuning. Thus, a 1 MHz detector has better than a 10:1 output of the IF section is amplified to produce almost a square advantage over a 10.7 MHz detector with respect to thermal drift. wave. The signal is then split into two signals, with one of them So, why are all receivers not double conversion designs with a routed through a phase shifting circuit. The signals are then detector at 1 MHz? For starters, double conversion receivers mixed back together with the phase of one signal delayed by 90 involve a lot more parts and cost more to build and align, since degrees (the signals are in quadrature). The average level of the there are two oscillators and two sets of IF filters. Secondly, two resulting signal is directly related to the phase shift (frequency oscillators can produce a lot more internal intermodulation variation) of the radio signal. problems, since the oscillators can leak into other circuitry and A digital pulse counting detector is a different, but much more even interact with each other, causing all kinds of “strange” effective method of converting an FM radio signal into an audio effects. Properly designed double and even triple conversion signal. The counting detector generates a stream of fixed-width receivers, although more difficult to design, offer better perfor- DC pulses at intervals controlled by the frequency of the radio mance in the final analysis. signal. COMPANDORS (expanders) VARYING DC = AC AUDIO SIGNAL The expander which follows the detector in a receiver must be a perfect “mirror image” of the compressor in the transmitter. Its purpose is to complete the noise reduction companding process by doubling the dynamic range of the audio signal, reversing the compression applied in the transmitter. The audio dynamics are compressed in the transmitter at a 2 to 1 ratio. The expander in the receiver expands the audio dynamics by the same ratio, to restore the original audio signal dynamics. (Compandors are 455KHz PULSES AT discussed in more detail in the section entitled “AUDIO SIGNAL FIXED PULSE WIDTH PROCESSING.” As the frequency increases, the pulses are spaced closer together. AM REJECTION As the frequency of the radio signal decreases, the pulses are The primary method of improving the AM rejection of a receiver spaced more widely. The average voltage level of the pulses in is to apply heavy limiting just ahead of the detector. The limiting any given time interval will fluctuate in direct proportion to the converts the signal into almost a pure square wave so that AM frequency of the FM signal, producing a varying low frequency level fluctuations will not change the shape of the waveform voltage (the audio signal). feeding into the detector. Counting detectors normally operate at frequencies under 1 Some types of detectors also provide AM rejection. A quadrature MHz, which means that they can only be effectively used in detector has no inherent AM rejection, whereas a pulse counting double conversion receivers. Trying to use a counting detector in detector does provide additional AM rejection. a single conversion receiver would mean that the oscillator frequency would be very close to the carrier frequency, in order AUDIO OUTPUT SECTION to mix down to a frequency low enough to allow the detector to The audio section of a receiver must provide ultra low noise gain, operate without distortion (remember how the superhet design with minimal distortion. It must also provide the correct output works). A first IF frequency close enough to the carrier fre- connectors, balanced or unbalanced configuration, and output quency (several hundred kHz) to allow mixing down to a levels for the intended application. Low cost receivers typically frequency of several humdred kHz would not allow adequate provide only a single output, generally unbalanced as well. High front-end filtering for the necessary image rejection (image quality, multi-purpose receivers will offer several outputs with rejection is discussed in the following paragraphs). Counting various connectors at different levels for use with a wide variety detectors offer extremely high temperature stability and inher- of sound and recording equipment. ently high AM rejection. Counting detectors are used in only the most advanced wireless receiver designs, as of the date of this writing.

10 Receivers

SQUELCH TECHNIQUES is off, and eliminates noise at turn-on and turn-off, but it does not The “squelch” circuitry in a receiver is employed to mute the address the issue of strong and weak signal conditions. audio output when the matching transmitter is turned off, or A unique technique called SmartSquelchTM is employed in some when signal conditions are too poor to produce a usable signal to Lectrosonics receivers. This is a microprocessor controlled noise ratio. Several different methods are used: technique that automatically controls the squelch activity by 1) Fixed RF level threshold monitoring the RF level, the audio level and the recent squelch- 2) Variable level controlled by HF audio noise ing history over a time period of several seconds. The system 3) Pilot tone control signal provides aggressive squelch activity during strong RF signal 4) Digital code control signal conditions to completely eliminate noise caused by multi-path 5) Microprocessor controlled algorithm (SmartSquelchTM) conditions at close range. During weak RF signal conditions, the system provides less aggressive squelch activity to allow Two opposite conditions require different squelch activity: maximum operating range by taking advantage of audio masking 1) Close operating range with a strong average RF level to bury background noise. 2) Distant operating range with a weak average RF level At a close operating range with a generally strong RF level, an INTERFACE ideal squelch would be aggressive and mute noise caused by With the advent of microprocessor control, a powerful tool is multi-path dropouts without allowing any noise to be produced in available to assist in identifying RFI and find clear operating the audio signal. The problem with this approach is that an spectrum. Software supplied with the Lectrosonics UDR200B aggresive squelch will reduce operating range significantly. receiver provides a graphical display of all internal settings and At longer operating distances with a lower average RF signal status, as well as adjustment of a variety of operating modes. ® level, an ideal squelch would be less aggressive and allow the RF Utilizing an RS-232 compatible PC interface for the Windows signal to dip closer to the noise floor in order to extend operating operating system, the receiver can be also used to perform a “site range. This approach, however, can allow brief “noise-ups” at scan” when setting up a wireless system in a new location. close range caused by multi-path signal conditions. A fixed RF level squelch systems monitor only the incoming signal level to determine the need to squelch. While the squelch threshold is often adjustable in this type of design, determining an optimum setting is difficult at best, since the average RF level in any given situation is almost impossible to predict. The receiver can also be falsely triggered by interference when the matching transmitter is turned off A squelch system that utilizes HF audio noise to control the squelch threshold is effective at muting the receiver when the transmitter is turned off. This approach also assumes that a dropout is preceded by a build up of high frequency audio noise. While this type of squelch is fairly effective in most cases, it can The lower section of the display provides a graphical, scanning also be “fooled” by audio containing a large amount of high spectrum analyzer for conducting site surveys. During scanning, frequency content such as jingling car keys or coins. the receiver is tuned in steps across its tuning range and markers A pilot tone controlled squelch system normally uses a continu- are place on the screen to indicate the frequency and signal ous supersonic audio signal generated in the transmitter to strength of signals found. control the audio output of the receiver. The receiver must be more sensitive to the pilot tone signal than the RF carrier to avoid inadvertent squelching when the carrier is weak, yet still strong enough to produce useable audio. This approach is highly reliable in muting the receiver when the transmitter is turned off, but it does not address the issue of strong and weak RF signal conditions when the transmitter is close or at a distance. A digital code squelch technique utilizes a supersonic audio signal containing a unique 8-bit code generated in the transmitter to signal the receiver to open the audio output when the transmit- ter is turned on. The code is repeated several times at turn on to ensure that it is picked up by the receiver. At turn off, the transmitter first emits another code to signal the receiver to mute the audio, then after a brief delay, shuts down the transmitter For use with multi-channel wireless systems, the software also power. A different code is used in every system to avoid provides a sumary screen showing the most critical, real-time conflicts in multi-channel wireless systems. This approach is activity for either 25 or 42 receivers simultaneously. RF and highly effective at keeping the receiver quiet when the transmitter audio levels, internal temperature and the status of batteries in the transmitters are all shown in a multi-colored display. Windows® is a registered trademark of Microsoft Corp. 11 Wireless Microphone Systems

12 Diversity Reception

DIVERSITY RECEPTION The term “diversity” is one of the most widely misunderstood A multi-path null is illustrated below. In this example, the signal concepts of wireless systems. This word stems from the root from the transmitter reaches the receiver antenna via a direct path word “diverse,” meaning “not correlated.” As it applies to and a reflected path. The reflected signal path is a bit longer than wireless microphone receivers, the term simply refers to the use the direct path, causing the two signals to be out of phase when of two antennas to eliminate “dropouts” caused by multi-path they mix together at the receiver antenna. The resulting weak phase cancellations (multi-path nulls). signal causes what is known as a “dropout.”

Direct Signal

The most common type of dropout might more appropriately be Dropouts occur when the transmitter and receiver antennas are in called a “noise up,” where the receiver audio output remains a particular location relative to each other. Moving the transmit- open during a multi-path null, and brief hiss, clicks, pops or other ter or receiver to a different location can oftentimes reduce or noise can be heard momentarily along with the audio. A eliminate the dropouts. Other objects that move around the complete loss of the audio can also occur if the multi-path null is room, like people’s bodies, also alter the reflected and direct deep enough to cause the receiver to squelch. VHF dropouts signals and can make dropouts either more or less prevalent. usually sound more like a momentary swishing or hissing sound, The wavelength of radio signal carriers at VHF frequencies sometimes along with a buzzing sound. UHF dropouts are more ranges from about 5 to 6.5 feet long. At UHF frequencies, the shorter in duration than VHF due the the higher frequency and wavelength ranges from about 12 to 20 inches. The point is that shorter wavelength, sometimes sounding more like a pop or the “dropout zone” (the location where a dropout occurs) will be click. larger at VHF frequencies than at UHF frequencies, so antennas Multi-path conditions that cause dropouts are very common have to be moved farther with a VHF system than with a UHF indoors, since the output of a wireless transmitter radiates in all system to alleviate dropouts. This also means that locating and directions and bounces off of many types of surfaces in the room. being able to identify a dropout zone during a walk test is a bit In reality, a wireless system operating in a room will be generat- easier with a VHF system than with a UHF system. ing perhaps hundreds of reflections around the room, but the system continues to operate since the direct signal is normally stronger. Metal is an especially good reflector, so multi-path conditions also occur outdoors, since the transmitter signal can be efficiently reflected from cars, trucks, trailers, metal building surfaces, etc.

13 Wireless Microphone Systems

In this simplified illustration of diversity reception, the signal The type of diversity reception circuitry chosen in the receiver arriving at antenna A is largely cancelled by a multi-path null, design includes a number of considerations, including cost, size leaving little signal left for the receiver. The signal at antenna B and weight, performance and the practicality of each circuit type remains strong and provides adequate signal for the receiver to for a given application. produce a usable audio signal to noise ratio. Cost is the major criteria for designs driven primarily by market Note that the illustration shows antenna B as a “remote” antenna considerations. Size and weight are most important in receivers connected with . The spacing between the antennas designed for field production. Performance is the primary focus must be at least 1/2 wavelength of the operating frequency to in high-end studio and stage receivers. In designs aimed at ensure that the antennas are receiving uncorrelated (“diverse”) motion picture production, the price of the wireless equipment is signals to gain the full benefit of diversity reception. far out-weighed by the cost of perhaps even a single day of Imagine what would happen if antenna B was also mounted on production, so audio and RF performance is the primary concern. the receiver. If the system was a VHF design, there would be a How the incoming signals from two different antennas are strong chance that the multi-path null would occur simulta- handled after they enter the receiver is what makes the difference neously at both antennas. What would be the benefit of trying to between a good design and one that has problems. Using switch back and forth between two antennas that are picking up diversity reception makes little sense unless the receiver is a high the same signal? The difference between the two signals would quality design to begin with. A low sensitivity diversity receiver be either nonexistent or so minimal that it would not have any will often have problems in an environment where a single effect on the reception. At UHF frequencies with the shorter antenna, high performance receiver will operate without noise or wavelengths, there might be enough space between the antennas dropouts. “Diversity” reception of any type will do little, if to achieve some benefit of diversity reception when two antennas anything, for a low performance receiver. In fact, it can even are mounted on the receiver. make matters worse. Diversity circuitry implemented in a high quality receiver with The following pages illustrate and discuss several different excellent sensitivity will reduce or eliminate multi-path dropouts, techniques used for diversity reception in various designs with and in some cases, increase operating range. The improvement varying degrees of success. in reception will vary depending upon the diversity methodology chosen by the designer.

14 Diversity Reception

PASSIVE DIVERSITY This is simply the addition of a second antenna to a single The logic behind using two antennas as opposed to one is simply receiver, placed ½ wavelength or more away. This can be “playing the odds.” The chances of a multi-path “null” occurring accomplished easily with an outboard combiner and a second at both antennas at the same time are perhaps less, but when both antenna. Two combined antennas will gather more RF signal and antennas have a good signal, but the signals are out of phase with can reduce dropouts to a small degree. each other, dropouts will still occur. Real world tests have shown that there is no real improvement in the prevention of dropouts by simply using two antennas.

Passive Diversity Method

FRONT AUDIO COMBINER IF SECTION AUDIO END AMP

ANTENNA PHASE SWITCHING DIVERSITY The primary advantage of this technique is small size, which The problem with this approach is: explains why this method is used in compact receivers designed 1) The receiver doesn’t react until it is already in trouble. for field production. Two antennas are mixed to feed a single 2) There is always the possibility that switching phase will make receiver, with a phase reversal switch added to the input of one of a marginal problem worse. the antennas. When signal conditions deteriorate, the phase of one of the antennas is reversed and logic circuitry then deter- 3) Since the switching circuitry is in the RF signal path and is mines whether or not the switching action has improved the triggered only at low RF levels, it can produce a “click” when signal to noise ratio or not, and decides whether to latch in that the switching occurs. position or switch and sample again. The antenna phase will A special adaptation of this diversity technique is a microproces- remain in the better position until the signal deteriorates again, sor controlled algorithm called SmartDiversityTM offered by when the process is repeated. Lectrosonics in compact receivers. Embedded firmware in the The logic underlying this approach is this: receiver controls the diversity sensing and switching circuitry based upon an analysis of RF level and the rate of change of RF 1) If either antenna has a strong signal, reception will be OK. level. The firmware determines the optimum timing for switch- 2) When both antennas have an in-phase weak signal, the signals ing and sampling to minimize dropouts and eliminate noise in the will add to each other and provide more total RF signal. audio that could be caused by the switching activity. This 3) When both antennas have a strong signal, but they are out of “intelligent” algorithm also integrates with the SmartSquelchTM phase with each other, phase cancellation between them will firmware in these receivers to employ “opportunistic” sampling reduce the signal level reaching the receiver. When this and switching. The system will take advantage of brief squelch- occurs, the receiver reverses the phase of one of the antennas, ing activity to switch, then sample and determine the best thus restoring the RF signal in most cases. antenna phase setting while the audio is being muted by the squelching system.

Antenna Phase Diversity Method

FRONT AUDIO PHASE IF SECTION AUDIO SWITCHING END AMP CIRCUITRY

LOGIC CIRCUITRY

15 Wireless Microphone Systems

AUDIO SWITCHING DIVERSITY This approach uses two separate receivers, selecting the audio The switch from one receiver audio output to the other must output of one of the receivers. It is quite effective at overcoming occur at the “zero crossing” point in the audio signal, or a click dropouts, but only provides a minor improvement in operating or pop will be heard. This means that additional circuitry is range. The switching action is usually triggered by comparing needed in the receiver to determine when the audio signal is at incoming RF levels and switching to the receiver with the the zero crossing point and enable the switch at that instant. In stronger RF signal, which usually produces a better signal to addition, the receiver audio output levels must be perfectly noise ratio in multi-path conditions. matched, or an audible change in level will occur when the This method is often called “true diversity” by some manufactur- switching takes place. ers, seemingly to discredit other diversity techniques and place a Some receiver designs handle switching so poorly that they particular receiver model in a more competitive position in the actually interrupt the audio signal briefly, or shift the audio level market. This technique is a valid approach to implementing a when the switch occurs. You can well imagine what that sounds diversity receiver, but suffers from some inherent limitations just like when the overall RF level is low or in severe multi-path like other diversity techniques. situations and the receiver is switching back and forth rapidly. First of all, the physical size of two receivers is greater than a Another problem with an audio switching design revolves around single receiver (common sense). Given that the physical the method of sensing used to trigger switching. If the switch is dimensions of the front-end filters and circuitry in a high quality activated by comparing RF signal strength in the receivers, it is receiver are limited to a certain minimum size due to the likely that the switch can be “fooled” by external RF signals at frequency wavelength, this design is impractical for compact one antenna. For example, if an RF noise source is near one of receivers intended for field production. In addition, two receivers the antennas, the receiver might think that this antenna has a require more power than one, which imposes another limitation stronger RF signal level and favor it in the switching. An to using this approach to diversity reception on a compact, external RF noise source produces mostly high frequency audio battery powered receiver. In order to implement this technique in noise in the receiver, so this type of design could have serious a compact, battery powered receiver, serious compromises must problems in a typical stage setup where synthesizers or other be made in the circuit design to reduce the physical size and digital equipment in use. If the receiver offers high selectivity, power requirements. then using the RF signal level at each receiver to control the This diversity reception technique is, however, very effective at switching is a safe assumption. minimizing dropouts in a receiver design that is not limited by While most of this discussion may seem negative toward this power requirements, physical size and cost. The logic behind approach to diversity reception, the fact is that this technique is this approach assumes that if the signal at one antenna is bad, one of the more effective methods of reducing dropouts when it then the signal at the other antenna is good. More often than not, is used in a high quality receiver design. this is the case, but there are still circumstances that occur where a poor signal exists at both antennas.

RF SENSOR

FRONT IF SECTION AUDIO END AMP

SWITCH AUDIO

FRONT IF SECTION AUDIO END AMP

RF SENSOR

ZERO CROSSING The “zero crossing point” in the audio signal occurs between the positive and negative voltage swings AUDIO WAVEFORM when the voltage is at zero.

16 Diversity Reception

RATIO DIVERSITY (audio ratio combining) This approach to diversity reception utilizes two separate Another advantage to the ratio diversity technique over other receivers, sharing a common oscillator and audio circuitry. The types resides in the fact that no hard audio switching is used in audio outputs of the receivers are used simultaneously, being the process. Since no switching occurs, there is no need for mixed by a “panning” circuit in a ratio controlled by the com- additional circuitry to enable switching during zero crossing in parative RF levels at the receivers. This method anticipates the audio signal. Output level differences in the two receivers are dropouts long before they occur, since the comparative RF level also less critical since no abrupt switch occurs. Lectrosonics sampling and mixing starts taking place at higher RF signal OptiBlendTM ratio diversity circuitry is damped to provide a levels than in other diversity designs. By the time the signal level smooth, seamless panning action that eliminates audible artifacts at one receiver drops low enough to produce noise, the panning of diversity operation. circuitry has long since shifted to the other receiver. When both receivers have a strong signal, their audio outputs are The ratio combining process is similar to an audio switching mixed together evenly. As the signal level drops, the panning diversity design, in that both techniques assume that if the RF process will begin to occur at about 40 uV. As the RF signal level at one receiver is low, it will be better at the other receiver. continues to drop, the panning action becomes more aggressive The difference between these two techniques rests in the fact that until the signal level is at about 1 uV, when the circuitry finally a ratio diversity receiver utilizes both receivers simultaneously, acts as a soft squelch and mutes both channels. As long as either whereas an audio switching type uses only one receiver at a time. antenna has at least 2 or 3 uV of signal, the receiver will continue This difference comes into play when the overall RF signal level to deliver a usable audio signal to noise ratio. is low and the receiver is struggling to find enough signal to produce a usable signal to noise ratio. In this situation, two antennas will gather more signal than one, and the panning circuitry in a ratio diversity receiver will continue to balance the outputs of the two receivers for the lowest noise even at very low RF levels. A switching diversity receiver will be busy selecting one or the other receiver, but not be able to use both receivers at the same time.

RF SENSOR

FRONT AUDIO IF SECTION END AMP

AUDIO PANNING AUDIO CIRCUIT

FRONT AUDIO IF SECTION END AMP

RF SENSOR

17 Wireless Microphone Systems

Sensitivity is another aspect of receiver performance to consider Comparative Squelch Thresholds before making an assumption that a diversity design is superior by default. In the example of Comparative Squelch Thresholds shown here, the signal from the transmitter dips to a level of 5 uV due to operating range and multi-path nulls, which is not at all uncommon. A receiver that squelches at 7 uV (threshold A) in this situation will shut down and mute the audio from this transmitter signal, no matter what sort of diversity circuitry is employed. A single antenna receiver that will work to 1 uV (threshold B), however, will continue to operate and deliver usable audio. It is always a good idea to check the sensitivity specification of any receiver being considered for purchase or rental along with the type of diversity circuitry being used in the design to get a good idea of how well the device might actually work. There are big differences from one manufacturer to another.

18 Audio Signal Processing AUDIO SIGNAL PROCESSING

Audio signal processing is used in almost all sound recording and reinforcement systems to match the equalization and dynamic range of the source material to the recording media or sound MAX AUDIO LEVEL DISTORTION (CLIPPING) WITHOUT system. Various methods are used for applications ranging from LIMITING motion picture optical soundtrack recording and music record- ings, to sound reinforcement and telephone systems. Wireless CLEAN LIMITING microphone systems are also subject to a maximum dynamic range to minimize noise and distortion, which requires several types of audio signal processing. AUDIO LEVEL

The dynamic range of an uncompressed audio signal coming MODULATION WITH LIMITING from a microphone with a live talker or instrument can easily be greater than a wireless system can cleanly handle. Without compression and limiting, the background noise inherent in any MIN wireless system will be audible. The background noise level -20 -10 0 +10 +20 +30 varies radically as the transmitter is moved around. When the audio signal is present at a fairly high level, the background noise INPUT LEVEL - dB is masked out by the audio. During pauses in speech or with low level audio, however, the background noise can become clearly There several good reasons to use an input limiter in the audio audible. In addition, very high input levels to the transmitter can stage of a wireless transmitter. First, government regulations produce distortion unless some form of clean limiting is provided restrict the maximum FM deviation that is allowed, regardless of in the transmitter. the input signal level. Secondly, if too much signal is delivered to the audio amp in the first stage, overload distortion (clipping) will The audio signal processing applied in wireless microphone occur. It is interesting to note that only a few manufacturers systems is employed to reduce noise and lower distortion. The include a limiter in the input stage, in spite of the fact that it is a signal processing includes several basic processes: valuable design “tool” that makes a significant improvement in the 1) Pre-emphasis/de-emphasis (utilized to increase the signal to performance of the system. noise ratio of the system) A good limiter in the input stage will improve the signal to noise 2) Input limiting (to minimize overload distortion) ratio of the system noticeably, and prevent distortion during input signal peaks. A good limiter will handle signal peaks about 12dB 3) Compandors (compressor/expander noise reduction) above full deviation. A better design will handle peaks up to 20dB. 4) DNR filtering (dynamic high frequency noise reduction) The best systems available will cleanly limit peaks over 30dB above full deviation at any gain setting. PRE-EMPHASIS/DE-EMPHASIS Input overload (clipping) is used to limit the maximum deviation in Most wireless microphone systems include a high frequency most designs. While this keeps the system legal, it produces gross boost (pre-emphasis) in the transmitter which is offset by a distortion and sacrifices the signal to noise ratio of the system. The complementary roll off (de-emphasis) in the receiver. The only way to guard against overload distortion without a limiter is to process is similar to simple noise reduction systems used in some lower the input gain so that signal peaks do not clip the input amp. tape recorders and typically improves the signal to noise ratio of The problem with this approach, however, is that the average signal the wireless system by about 10 dB. level is then too low to produce a good signal to noise ratio in If excessive pre-emphasis is applied, it is possible to generate normal operation. This is one of the reasons why background distortion (high frequency sibilants) caused by the IF filters in a noise is often audible and dropouts occur frequently in some narrowband receiver during full modulation. Wideband IF filters designs. reduce or eliminate this problem at the expense of selectivity. In some circuit designs, the limiter range varies with the gain control setting so that the limiter acts directly on the input gain INPUT LIMITING circuit. This restricts the limiter range to being roughly equal to A distinction must be made between the input limiter and the the amount of gain set on the control. In other words, the limiter compressor that is part of the overall compandor circuitry. These simply turns the gain control down during loud peaks. This are two separate circuits that operate differently and are used for approach creates a real problem with a loud vocalist, precisely different purposes. The input limiter is a circuit used in the when clean limiting is needed most. In this case, the input gain transmitter input circuitry to put a “ceiling” on the maximum control will usually be set at minimum, which means that there is signal level to keep the gain amplifier from overloading and to keep little or no limiting available, since there is no gain set on the the maximum deviation within legal limits. The circuitry in the input. The best limiter designs utilize a separate circuit stage compandor’s compressor, on the other hand, is used as part of an independent of the input gain circuitry. overall noise reduction process, complemented by a mirror image expansion process in the receiver. The input limiter is at the transmitter input, followed by the compressor.

19 Wireless Microphone Systems

COMPANDORS The perceived signal to noise ratio of a wireless mic system is There are many boastful claims made by a number of manufac- very much improved with signal processing circuitry called a turers centered around their particular companding circuitry. “compandor.” The term “compandor” is a combination of the The compandor is critically important to the audio quality the words “compressor” and “expander.” wireless system produces, but only if the rest of the wireless (the spelling of “compandor” with an “or” was defined by the company who system is a high quality design. It is of little importance how manufactured some of the first IC chips for the telephone industry) good the audio processing circuitry is if there are problems with the RF performance and the system is suffering from dropouts or Companding is a two-fold process that depends upon a “mask- interference. In addition, poor RF performance can produce ing” effect to improve the signal to noise ratio of the wireless anomalies in the RF link that generate audio signals in the system. Masking is a process of burying (or covering up) one receiver that did not exist in the transmitter. sound with a louder one, taking advantage of the way a human ear hears things. Masking works in a wireless mic system by Excessive high frequency noise generated in the RF link can taking advantage of the fact that when the audio level is high cause the compandor to mis-track. This noise appears in the enough, the ear will not hear the underlying background noise receiver along with the audio signal, but was not present in the that is generated in the RF link. original audio signal fed into the transmitter. Much of the noise produced by a poor RF link is supersonic, and while not audible, The compressor action in the transmitter operates around a it can modulate the compandor, causing “pumping” and “breath- reference level, operating to keep high level signals lower and ing” effects in the audible sound. In the best designs, an active low level signals higher. The effect of this compression is to filter circuit in the receiver just ahead of the compandor provides reduce the dynamic range of the audio signal, effectively raising sharp high frequency roll off to minimize mistracking caused by the average level. Raising the average audio level farther above high frequency noise in the audio signal. the background noise provides a dramatic improvement in the signal to noise ratio of the RF link in the system. One of the fundamental considerations in compandor design is the effect of different attack and decay times used in the compres- Compandors were first used in the telephone industry to provide sor and expander. The attack and decay times must be carefully noise reduction for long distance telephone lines where excessive controlled to optimize the dynamic action for the intended noise buildup could be as loud or louder than the audio signal application. The compressor in the transmitter and the expander itself. Compandors are also used in analog tape recording in the receiver must track in a perfectly complementary manner. processes to overcome tape hiss. An immense amount of time A compandor idealized for high frequency program material will has been spent in engineering to improve the companding process cause excessive distortion of low frequencies, since the gain will for many applications. be affected by the voltage level changes on each cycle of the low Compandors for wireless mic systems use a 2:1 ratio. The frequency audio waveform. A compandor with slow attack and compressor in the transmitter reduces the dynamic range by 2:1, release times will reproduce low frequencies without distortion then the expander in the receiver reverses the compression at a (ideal for the lower frequency fundamentals of bass guitar, for complementary 1:2 ratio to restore the original dynamics of the example) but it would then not be fast enough to accurately track audio signal. In looking at the drawings below, notice that before the fast rise times of higher frequencies. So, what occurs is compression, the lowest audio level is only 40 dB above the noise perhaps a compromise in the design process. floor. After compression, the lowest audio signal is 60 dB above the noise floor, providing a 20dB improvement in the signal to noise ratio. The compressed audio signal is used to modulate the carrier to provide a significant improvement in the signal to noise ratio in the RF link.

FIRST HALF OF COMPANDING PROCESS SECOND HALF OF COMPANDING PROCESS (2:1 COMPRESSOR IN TRANSMITTER) (2:1 EXPANDER IN RECEIVER)

+10 +10 +5 +5 0dB 0dB (REF) COMPRESSED COMPRESSD (REF) DYNAMIC RANGE DYNAMIC RANGE ORIGINAL AUDIO ORIGINAL AUDIO DYNAMIC RANGE DYNAMIC RANGE -20 -20

-40 60dB -40 60dB

40 dB 40 dB

-80 NOISE FLOOR NOISE FLOOR -80 NOTE: The specific values shown in this drawing are only used to depict the concept of a compandor. These values will vary significantly in various designs and at various transmitter input signal levels. 20 Audio Signal Processing

DNR FILTERING If the compandor has a fast attack time and a slow release time, As a finishing touch to a complete audio signal processing brief transients in the audio signal will control the compandor package, DNR filtering (dynamic noise reduction) is added to gain for relatively long periods of time. The resulting problem is provide an optimum signal to noise ratio during poor signal “breathing,” which is residual background hiss heard following conditions. each word or sound. The cure for this is a faster release time, A dual-band compandor provides excellent, low distortion noise however, this increases low frequency distortion because the reduction, but an anomaly can still occur under special conditions. circuitry will then act on the trailing side of a low frequency When the only audio signal present consists of a low level, low waveform, causing distortion. frequency transient, an effect called “breathing” can still occur. A conventional compandor with a single attack time and a This happens when high frequency noise in the environment or reasonable release time for the entire audio spectrum will still the RF link can be heard briefly following a low frequency audio increase system distortion about 0.5% at 200 Hz and about 1% at transient. In this situation, the bass compandor applies a longer 100Hz. There is no way around this problem in a conventional time constant, allowing a brief “noise tail” to occur following the compandor. audio. This effect is most noticeable when the audio signal is at a The problems with high and low frequency distortion have given low level, at a low frequency and the listening environment is rise to a new generation of compandors, where high frequency very quiet (ie. with used in a studio setting). All components of the audio signal are processed separately from compandor designs exhibit this breathing effect to one degree or low frequency components. This process, called “dual-band another under these conditions. This is not a unique problem companding,” is only found in the most advanced wireless mic with a dual-band compandor. systems, since it adds considerably to the cost of engineering and To suppress noise in this situation, a unique dynamic filter circuit manufacturing the system. is employed. DNR consists of a dynamic low-pass filter with a The dual-band companding process utilizes crossover network, variable hinge point that automatically reduces high frequency somewhat like a speaker system. The audio signal is separated noise during poor signal conditions. The hinge point frequency into high and low frequency bands, and then each band is of the filter is controlled continuously by an analysis and combi- processed with its own idealized attack and decay times. The nation of the RF level, the audio level and the high frequency high frequency section utilizes faster attack and decay times content of the audio signal. appropriate for the typical signals it handles, and the low frequency section operates with longer time constants for the typical signals it handles. There is also interaction between the Frequency Response with DNR Filtering two compandors to insure linearity in the audio signal. Dynamic Hinge Point LEVEL

Hz 30 100 1kHz 10k 20k Low Frequency High Frequency (slower attack and decay) (faster attack and decay) When weak RF signals and low level audio are present, the hinge point of the filter moves to a lower frequency to roll off high frequency noise. This is a single-ended process that occurs entirely inside the receiver. 201k 20k Hz DNR utilizes the masking effect somewhat like a compandor. Note that the compandor acts only on the dynamics of the audio When audio is present, even at a moderate level, or audio with signal, without affecting the frequency response of the overall high frequency content is present, the background noise will be system. The plot shown above only depicts the separation in masked, so the filter moves completely out of the way to avoid control signals that affect the compandor action. In the dip altering the original audio signal. The DNR circuit is extremely shown at 1 kHz, both high and low frequency sections of the sensitive and responsive, and does not alter the natural frequency compandor circuit control the dynamic action equally. response of the overall wireless mic system. TESTING FOR AUDIO PERFORMANCE Some excellent tests of audio performance are outlined in a chapter in this Wireless Guide entitled “Evaluating Wireless Microphone Systems.” The tests described in this section challenge even a high performance wireless mic design, and provide dramatic demonstrations of the design challenges and performance issues described in this chapter.

21 Wireless Microphone Systems

22 Antennas

ANTENNAS The frequency of the carrier determines the “wavelength” of the HAND-HELD TRANSMITTER ANTENNAS radio signal. Radio signals and light travel at the same speed. If Most hand-held transmitters utilize an internal antenna which you take the distance that a radio signal travels in one second, radiates the RF signal through a plastic housing. A few hand- and divide it by the carrier frequency, you will derive the actual held transmitter designs use a metal housing, which becomes part length of one cycle of the carrier frequency. Most wireless mic of the antenna circuit. It is also common to employ flexible whip transmitters and receivers use “quarter wave” antennas. This antenna protruding from the bottom of the housing, since the means that the ideal length of the transmitter and receiver whip is only a few inches long at UHF frequencies. antennas would be 1/4 of the total wavelength. Hand-held transmitters benefit from the fact that they are held Transmitters are normally held in the hand or worn on the body out in the air with only the user’s hand in direct contact, allowing so there is always an effect on the antenna caused by the user’s the antenna to radiate efficiently. In designs that use an external body. Transmitter antenna designs vary widely on units offered whip, the user’s hand becomes part of a quasi-. by various manufacturers. A dual tapered, “hour glass” shape works very well for a hand- Receivers are often mounted on other equipment or near large held transmitter. It provides a secure grip and is comfortable to metallic surfaces, any of which can affect the efficiency of the hold in the hand. When held with one hand, the antenna is antenna. A , usually a quarter wave, mounted generally clear to radiate the signal efficiently, whether the hand directly on a receiver chassis uses the receiver housing to provide is positioned on the top or bottom of the transmitter. Held with a ground plane for the antenna. Remote antennas of various two hands, however, with one at the top and one at the bottom of types are generally used in critical applications, or for maximum the transmitter, the antenna output can suffer. operating range. Rather than go into the intricacies of antenna theory and design, ANTENNA ANTENNA ELEMENT ELEMENT the following information simply highlights the major concerns and operational benefits of the most commonly chosen designs. BELT PACK TRANSMITTER ANTENNAS VHF belt-pack antennas normally use the shield of the micro- phone cable as the antenna, although some older designs still RF INTERNAL INSULATORS PC BOARDS exist with a “dangling wire.” UHF belt-pack designs normally utilize a separate 1/4 wavelength whip to maximize the radiated output. It is important to keep in mind that the radiated output of “PLUG-ON” TRANSMITTER ANTENNAS the antenna will be reduced when it is placed on the user’s body, “Plug-on” transmitters with integral antennas utilize the housing leaving less radiated signal to reach the receiver. Generally around the battery compartment as one antenna element, with the speaking, it is a good idea to positon the transmitter with the microphone body and user’s hand forming the other half of a antenna oriented vertically, to produce a circular radiation dipole configuraton. An insulator just below the input coupler pattern. In a vertical orientation, the user can move about at will, separates the antenna elements. The radiated RF power from this but still radiate a strong enough signal to reach the matching configuration is as high or higher than standard hand-held receiver. transmitter configurations.

MIC Antenna Element ANTENNA

Microphone Insulator

TYPICAL UHF ANTENNA BELT-PACK TYPICAL VHF TRANSMITTER BELT-PACK TRANSMITTER

MIC

23 Wireless Microphone Systems

RECEIVER ANTENNAS There is little that a user can do to significantly alter the effi- Whip antennas can produce problems in multi-channel wireless ciency or design of a transmitter antenna. With respect to systems when the receivers are close together. In most applica- receivers, however, there are a number of alternatives to the tions, however, a whip antenna is generally sufficient when antenna supplied with the receiver. A wide variety of different operating up to three or so wireless systems in the same room. It types of antennas are available from a variety of sources, is best to use an RF multi-coupler and external antenna/s for any including specialty manufacturers who make only antennas. The multi-channel system consisting of more than three channels in purpose of this text is only to highlight a few general classes of the same location. antennas that serve some usefulness in operating wireless microphone systems. Rather than attempt to discuss the complex Helical style (rubber duckie) topic of antenna theory, simplified descriptions of some basic Similar to a conventional whip antenna, this type offers a shorter types are offered, including a few suggestions of which types physical length, but exhibits a more limited bandwidth than a 1/4 might serve certain applications. wave whip. Instead of extending the element out a 1/4 wave- In some applications, the performance of the antenna may be less length in a straight line from the connector, a helical whip is important than the physical limitations of the installation. For wound into a coil. The wire in the element is the same electrical example, in a boardroom it is usually important that the compo- length as a straight whip, but the overall physical length is shorter nents in the sound system be virtually invisible. Placing a high and more flexible after being coiled. A helical antenna is usually performance antenna on one of the walls of the boardroom is not as efficient as a straight 1/4 wavelength whip, but in some simply not permissible. The efficiency of the antenna is usually applications the durability of a helical design is more important not a problem, since the distance from transmitter to receiver than the efficiency. antenna is normally quite short. So, an antenna that blends into The most common application for helical antennas is with the decor, or is invisible, makes sense for this application, at the camera mounted receivers used for field video and film produc- expense of using less than an ideal type. tion, where a longer antenna could easily be broken or might get In other applications, such as theatre and stage, the performance in the way of other equipment. of the antenna is the primary concern, while the physical appearance of the antenna is of little or no importance. In this FLEXIBLE WIRE COIL example, the antennas may have to be located several hundred WRAPPED IN feet from the stage. In addition, a fair amount of RF noise or INSULATION interfering signals can be present at the antenna location. So, what makes sense in this application is to utilize a high effi- ciency, directional antenna, regardless of its appearance. The antenna is out of sight of the audience, so it makes little differ- ence what it looks like. The directional nature of the antenna can provide a strong RF signal from the stage, and also provide rejection of interfering signals coming from other directions. Ground Plane 1/4 wave whip This type of antenna provides significant gain over a 1/4 wave This is the antenna that is normally supplied with a wireless mic whip. It is constructed of a single, 1/4 wavelength vertical receiver. The whip can be a fixed length, or it can be a telescop- element surrounded by radial elements around its base. The best ing design. The length is approximately 1/4 wavelength of the impedance match is provided when the radials are angled operating frequency of the system. The whip functions as part of downward from the vertical at a 45 degree angle. The antenna a “quasi-ground plane” arrangement, with the whip working as should be positioned so that the radials are closest to the nearest the radiating element, and the receiver housing working as the boundary (floor or ceiling). Used outdoors, it ground plane. For most applications, a 1/4 wave whip mounted should normally be mounted vertically (the directly on the receiver provides adequate operating range. single element pointed upward). If it is mounted Oriented vertically, it provides a circular shaped pattern with on the ceiling of a room, it will generally work equal sensitivity in all directions except straight up and down. best inverted, with the single element pointed downward, so that the radials are closest to the ceiling. A ground plane antenna exhibits a circular coverage pattern, perpendicular to the vertical

FIXED ROD element. This would be a good choice for applications such as in a centrally located

1/4 WAVELENGTH position in a concert hall or large room away from significant sources of RFI. TELESCOPING ROD

24 Antennas

Coaxial LPDA This is a special type of antenna that can be made from common This abbreviation stands for “Log Periodic Dipole Array.” This coaxial cable. It functions much like a dipole antenna. The is a multi-element, directional antenna that operates over a wide center conductor is cut to 1/4 wavelength and protrudes beyond frequency range (such as 500 to 800 MHz) with a typical gain of the shield of the coax. The shield is also cut to a 1/4 wavelength 4 dB over a dipole antenna. The pickup pattern is similar to a and is folded back over the coaxial cable for a better impedance cardioid microphone, perpendicular to the radial elements. match to the receiver. A log periodic antenna is commonly used as a single antenna A coaxial antenna can be positioned vertically or horizontally. feeding a multi-channel wireless system which is operating on a While this type of antenna is not particularly efficient, it does variety of frequencies. An LPDA antenna should be mounted so permit mounting in awkward installations, such as above a that it is not close to a nearby reflecting surface. At UHF suspended ceiling or concealed along a wall. When positioned frequencies, there is usually not a problem with placement, but at vertically, it provides a circular coverage pattern for transmitters VHF frequencies there can be some limitations in indoor use. located perpendicular to the axis of the antenna.

LPDA antennas 1/4 WAVELENGTH SHIELD CENTER CONDUCTOR RF CONNECTOR

SHRINK TUBING 1/4 WAVELENGTH

Dipole “Rod style” This is a dual element antenna that exhibits a circular pattern. Each element is normally cut to 1/4 of the wavelength. The maximum sensitivity is achieved with the transmitter located perpendicular to the elements. Since a dipole antenna is easy to construct, it is used in many applications ranging from a concert “Shark fin” hall to an outdoor production scenario. Shown here is a versatile dipole design with tunable elements. A scale along the body of the antenna is marked for frequencies from 500 to 800 MHz. The elements are folded along the body Yagi for frequency adjustment, and to make the unit more compact for This is a multi-element, directional antenna that operates over a storage. limited frequency range. A Yagi antenna is a “parasitic array” of dipole elements. It consists of a basic dipole element modified by other dipole elements (the parasitic elements) which are placed at specific distances in front of and behind the dipole element. The element behind the dipole is called the “reflector” and the elements in front of the dipole are called “directors. As more “director” elements are added in front of the dipole, the pattern becomes more directional. A typical 3-element Yagi will produce about 3 or 4 dB of gain over a dipole. A 5-pole design can produce as much as 10 or more dB of gain over a dipole. The higher the gain, the more critical the placement.

minimum sensitivity

maximum sensitivity

transmitter

25 Wireless Microphone Systems

26 Synthesizers vs. Crystal Contolled Systems

FREQUENCY SYNTHESIS

Synthesized wireless microphone systems have become increas- 702.625 MHz. This control signal is filtered to remove digital ingly popular in recent years. Except for some very inexpensive noise, amplified and then applied to the voltage controlled systems intended for the consumer market, a clear majority of all oscillator. new systems now being introduced are synthesized. Wireless As the voltage controlled oscillator frequency closely approaches users are attracted to the idea that they are getting more for their 702.625 MHz, the phase/frequency comparator output will money and feel empowered by the ability to rapidly change to a change to a phase control signal. After some period of settling new operating frequency. Dealers like the fact that they can sell time, the two 125 kHz signals into the phase/frequency compara- the equipment they have in stock; it is not necessary to wait for tor will be locked in both frequency and phase. When this special order systems on custom frequencies. Synthesized occurs, the voltage controlled oscillator output frequency will be wireless microphones are also popular with field news crews, an exact multiple of 125 kHz, and as accurate in frequency as the location film and TV production companies, touring groups and reference oscillator itself. other wireless users who frequently travel. The term “frequency agility” (referring to synthesized equipment) has become one of Changing the programmable frequency divider’s divide ratio will the common buzzwords in the professional audio industry. move the synthesizer output frequency in steps of 125 kHz. For example, if the programmable frequency divider is changed to Despite their undeniable attraction, synthesized wireless micro- divide by 5622 instead of 5621, the voltage controlled oscillator phone systems have some significant limitations and shortcom- frequency changes to 5622 times 125 kHz, or 702.750 MHz. A ings, and sometimes introduce new problems while solving old divider ratio of 5623 yields 702.875 MHz, 5624 yields 703.000 ones. For example, synthesizers often affect audio quality quite MHz, and so forth. In theory, the output frequency range of the significantly, yet the connection between audio performance and synthesizer is limited only by the range of the voltage controlled synthesis is not apparent to most wireless users. Synthesis also oscillator and the available divide ratios of the programmable affects many other areas of wireless mic performance, as well as divider. As a practical matter, synthesizer noise performance and battery life, size, weight and cost. Because synthesis is a mixed other considerations normally limit the useful range to two or so blessing, a basic understanding of the technology will be very TV channels at VHF and roughly four to eight TV channels at helpful in making informed choices when selecting and using UHF. wireless mics. The phase/frequency comparator also has a lock detector output FREQUENCY SYNTHESIZERS that signals when the synthesizer has achieved stable on- All synthesized wireless microphone equipment use a phase frequency operation. In transmitters, the lock detector signal is locked loop circuit of some type to control the operating fre- used to turn on the transmitter output control switch, allowing the quency. In this type of synthesizer, the output frequency is unit to begin transmitting RF. Until the synthesizer is locked, the generated by a voltage controlled RF oscillator, usually operating output frequency might be anywhere within the transmitter’s directly at the desired output frequency. The voltage controlled tuning range and rapidly changing in frequency. Transmitting oscillators in wireless transmitters often also have a second before the synthesizer is locked on frequency can cause severe control input used to FM modulate the output signal. Some more interference to other equipment. complex synthesizers, however, use a separate FM modulator Basic phase locked loop synthesizers at lower RF frequencies are circuit. relatively simple to design using standard integrated circuits, at The output frequency of the voltage controlled oscillator is least if modest performance is acceptable. The high performance controlled by locking it to a very stable reference crystal oscilla- designs needed for professional wireless microphone systems are tor. This is accomplished by digital frequency divider circuits, a considerably more challenging, especially so at UHF. Careful phase/frequency comparator, control signal filter and control tradeoffs must be made between noise, audio frequency re- signal amplifier. The stable reference frequency is divided down sponse, low frequency distortion, lock-up time, power consump- to a frequency equal to the desired synthesizer channel spacing. tion, tuning range, spurious outputs, frequency step size and That is, if the channels are to be spaced at 125 kHz intervals, the several other inter-related performance factors. Users of synthe- output of the reference frequency divider will also be 125 kHz. sized wireless systems should look beyond the appeal of A high reference frequency and a divider are used because 125 synthesis to make certain that overall performance meets their kHz crystals are not stable enough or too large for use in wireless needs. mic equipment. SYNTHESIZER PERFORMANCE ISSUES The output of the voltage controlled oscillator is also divided Synthesizers have a major impact on audio signal-to-noise ratio down to 125 kHz by a programmable frequency divider. For (SNR). Because some type of audio processing (companding) is example, if an output frequency of 702.625 MHz is desired, the used in the vast majority of wireless mic systems, the effects of counter will divide by 5621 (702.625 divided by 5621 is 0.125). this are not always immediately obvious. In addition, specifica- The outputs of the reference divider and the programmable tions based upon static measurements do not ordinarily reveal the divider are then both applied to the phase/frequency comparator. problem. Unfortunately, some wireless manufacturers rely on Initially, the voltage controlled RF oscillator will not be at audio processing to conceal the effects of various design exactly 702.625 MHz. The phase/frequency comparator will compromises, including those in the synthesizer. A poor system output a control signal that will drive the voltage controlled SNR will be clearly revealed, however, by annoying “fizzing” oscillator up or down in frequency as necessary to bring it to

27 Wireless Microphone Systems sounds or “noise tails” at the end of words. This effect is also relatively close to the transmitter frequency. In most cases, there sometimes called “breathing”. The problem is especially will be spurious outputs above and below the carrier by an pronounced with certain kinds of background sounds are present, amount equal to the channel spacing. such as waves breaking, train and subway rumbles, elevator noise That is, if the transmitter output frequencies are spaced at 100 and heavy breathing by the performer. kHz intervals, spurious signals 100 kHz above and below the Although synthesizer noise is not the only cause of this problem, carrier will be present. This is often surprising to those accus- it is one of the most common. The culprit is phase noise, tomed to crystal-controlled transmitters, where close-in spurious sometimes also referred to as phase or frequency jitter. Both signals are frequently an indication of problems. Synthesized transmitter and receiver synthesizers are subject to the problem, transmitters sometimes also have spurious outputs above and and the phase noise of each is additive. The receiver FM below the carrier at the synthesizer reference oscillator frequency, demodulator cannot distinguish the synthesizer phase noise from typically 3 to 10 MHz. Both types of transmitters will also have the desired audio modulation, resulting in low level noise being at least small spurious outputs at the harmonic frequencies. Well- introduced into the audio. This artificial noise “floor” causes the designed synthesized transmitters generally have fewer spurious “noise tails” at the end of words, as well as audible level discrep- outputs than a crystal-controlled transmitter and the level of ancies. spurious signals will usually be lower. For a number of reasons, synthesizers have far more phase noise Synthesizer turn-on and turn-off can create severe problems than crystal-controlled oscillators. The phase noise also rises when several systems are in use. Almost all synthesizers sweep with increasing frequency, making UHF systems more suscep- across a wide range of frequencies upon initial turn-on. Some tible than VHF systems. To partially offset this, UHF systems amount of time is required for the digital circuitry to “lock” the normally have wider deviation (increased modulation) compared output frequency to the correct value. If the transmitter synthe- to VHF systems. The design of very low phase noise synthesiz- sizer sweeps across another active wireless channel when turned ers at UHF frequencies is quite challenging, and good designs on, the second system will experience severe interference. A tend to be more complex and expensive than those with lower maximum audio level “thump” is the most common result. performance. Phase noise also affects channel spacing; high Synthesized transmitters are required to disable the RF output performance circuits with 25 kHz channel spacing are usually until the synthesizer is properly locked. However, some units considerably more difficult to implement and more expensive have been found that do not have the disable circuitry, or do not that those with 100 kHz or more spacing. reduce the output sufficiently to prevent interference. Unfortu- Synthesized equipment is typically more susceptible to mechani- nately, it is quite possible for a transmitter to meet the minimum cal shock and vibration than are crystal controlled designs. FCC isolation requirements and still be capable of causing this Unless specific preventative measures have been taken, handling problem, especially when it is close to a receiver. Some designs or bumping a synthesized transmitter or receiver is more likely to also create momentary interference when turned off, with RF cause audible “thumps” in the system output. Even then, the output continuing for a short time after the digital circuitry has transmitter and receiver enclosures need to be more rigid and lost frequency control. This problem is serious, so it is essential designed so as to minimize coupling of the shock and vibration to verify that synthesized wireless transmitters turn on and off to the synthesizer circuitry. Electrical transients also cause cleanly before using them for a professional application. serious problems; battery powered units must be designed to prevent noise on the dc lines due to loose battery contacts. RECEIVER PERFORMANCE Receivers operating from ac lines must have sufficient filtering Another important consideration in wireless microphone systems and regulation to prevent power line transients and noise from is receiver selectivity and interference rejection. A synthesizer reaching the synthesizer circuits. can easily tune a receiver’s center frequency over a wide range. Power consumption for synthesizers is always considerably For the receiver to work properly, the RF filters at the input of the higher than for crystal-controlled circuits. Although great strides receiver must somehow cover the entire synthesizer tuning range. have been made in low-power high-speed digital circuitry, The most common approach is to simply to make the receiver’s synthesizer current drain is still considerably higher than RF filters very wide, accommodating the required range. This desirable. This is especially true of UHF equipment, which almost invariably compromises the receiver’s selectivity and its requires very high speed frequency divider circuitry. It is not ability to reject unwanted RF signals that can cause interference. unusual for the synthesizer in a high performance UHF transmit- The wider the tuning range, the more serious the performance ter to represent 35 to 50% of the total current drain of the unit. compromise. The digital circuitry also requires regulated operating voltage, The preferred approach is to equip the receiver with electroni- often at the expense of efficiency and a further reduction in cally tunable RF filters, especially if a relatively wide tuning battery life. range is desired. While this can work quite well, it does have a Synthesized transmitters will have spurious outputs, as do crystal number of drawbacks, cost being high on the list. The extra controlled transmitters. However, except for harmonics of the component costs are significant and the initial alignment can be transmitter output frequency, spurious outputs will be quite time consuming. Filter tuning versus the control signal is different for the two types of units. Crystal-controlled transmit- virtually always nonlinear, necessitating circuitry to store the ters typically have numerous low-level output spurs, usually required control signal magnitude for each frequency and separated from the carrier by 10 MHz or more. Synthesized generate the correct value signal. transmitters almost always have low level spurious outputs

28 Synthesizers vs. Crystal Contolled Systems

One common type of electronically tunable RF filter uses systems are in use. That is, if a transmitter should momentarily varactor (voltage variable capacitance) diodes as the tuning output a signal at each of the intermediate steps, the potential for element. Unless very well implemented, this type of filter can interference is very high. There can even be problems with compromise performance is several ways. One problem is that receivers, as a brief period of unwanted audio can occur during some varactor diodes, especially inexpensive ones, have internal tuning. Unless all frequencies will remain unchanged during a losses that can limit filter selectivity. This is a particular problem performance, it is essential that the transmitter have provisions to at UHF, where small component losses can greatly affect filter inhibit output until the final frequency is reached. There is performance. The result can be that there is no actual advantage particular risk for ENG use, as several types of equipment might over wideband non-tuned designs. be in use, there is no overall control and setup time is short to Varactor-tuned RF filters can also be overloaded by strong RF non-existent. signals, a very common situation with wireless microphone LCD displays on transmitters offer an intuitive appeal with a systems. This occurs when the voltage generated inside the filter direct frequency, group, etc. readout, but considering that by strong RF signals overwhelms the applied tuning voltage. transmitters are the most “handled” part of a wireless system, When this happens, the filter tuning changes and its selectivity is they can also introduce a problem with fragility. Simple screw- degraded. Sometimes, the filter tunes away from the desired driver-adjust subminiature rotary switches offer ruggedness, low signal and towards interference, seriously compromising system cost and reasonable ease-of use. In addition, unlike units with performance. Filters using other types of tuning components, LCD displays, the switches can be adjusted with the transmitter such as PIN (switch) diodes can perform much better in this turned “off”, eliminating any potential interference problems that situation. could occur while changing frequencies. Switches also provide It is possible to improve the performance of receivers with rapid access to widely spaced frequencies, as it is not necessary wideband filters by using special overload-resistant RF amplifi- to step or scroll sequentially thru all intermediate selections, as is ers. Because of the high power consumption of these amplifiers, true of pushbutton type controls. the approach is only feasible with larger ac-powered receivers. Displays for receivers are considerably more useful, especially Even with sophisticated circuitry, however, performance of those used for studio or fixed installation applications. In such wideband and tunable-filter receivers will still not match that of a use, it is not uncommon to use more than one transmitter with a quality fixed-frequency design. For this reason, only high particular receiver. The display lets the audio team keep track of quality, professional synthesized equipment should be used for which transmitter will be received. In addition, the risk of demanding applications. interference to other systems is minimal and the operator can mute any unwanted audio. The display is also useful for other OPERATOR INTERFACES purposes such as transmitter low battery warnings and other Typically, the programming interface to frequency synthesizers is miscellaneous system information. Finally, a front panel receiver in the form of a long binary word, or even two shorter binary display is particularly useful if the receiver is being remotely words. This must be translated to a form that is meaningful to controlled. the user. Several approaches have been used; some simple and some relatively complex. On the complex end of the scale, some OPERATIONAL ISSUES synthesized wireless transmitters and receivers use an on-board One of the primary attractions of synthesized wireless micro- microprocessor to drive a LCD display. Operating frequency can phone systems is the ability of the user to quickly change be displayed directly in six-digit form, such as “702.625”. frequencies. In many applications, this is also one of the primary Frequency can be changed via “up” and “down” pushbuttons, on disadvantages. In fact, several organizations with large numbers a small control panel, or perhaps inside a cover or in the battery of wireless systems will not use synthesized equipment unless it compartment. Generally, some form of electronic lock or cover can be “locked” to prevent field changes or restricted to a small is necessary to prevent accidental frequency changes. pre-established set of “allowed” frequencies. The problem is that Some equipment uses a “channel and group” approach. In this maximizing the number of available wireless frequencies case, based upon some rationale, the manufacturer designates requires careful frequency coordination and firm control of specific frequencies as being a certain channel within a desig- frequency utilization. Just one or two synthesized systems and nated group. The number of available groups, as well as the an undisciplined operator can easily cause very serious interfer- number of channels within the groups, varies from manufacturer ence problems in such an environment, often with costly to manufacturer. In particular, the actual frequencies represented consequences. by channel/group numbers is almost always completely different On a smaller scale, any organization using more than two or from manufacturer to manufacturer. This tends to make the three wireless systems can run into a small-scale version of this approach very unwieldy if equipment from more than one problem. This is especially true in major metropolitan areas and manufacturer is in use at a location, and a serious problem if a when the wireless is not under the control of a single person or a chart of actual frequencies versus channel/group numbers is not small team. In such situations, the ability to change frequencies available. is the ability to interfere with another system. In particular, the Some equipment offers the choice of direct frequency display or frequency relationships that cause intermodulation are not at all channel/group display. In either case, it is usually necessary to obvious, so random frequency changes always have the potential step thru all the intermediate choices to get to a desired new to create serious problems for another system. choice. Not only can this be time consuming, it can be a serious problem if frequency changes are required while other wireless

29 Wireless Microphone Systems

Synthesized systems can also be a problem for ENG and similar applications. There is no central control of frequency assign- ments and pressure situations are routine. When several ENG crews are present, a late arrival or a simple frequency change to improve reception can create serious problems. Suppose wireless user “A”, hoping for a little more range, changes frequency, thereby creating an intermod problem for user “B”. “B” then changes frequency to avoid the intermod, causing interference problems for users “C” and “D”, in turn causing both to change frequency themselves. Now “C” causes new problems for “A” and “D” creates a problem for “B”. This kicks off another round of frequency changes, possibly all while an important event is occurring and no one is getting quality wireless audio. While there might not be a really good answer to interference between users from different organizations, madly changing frequencies during the event is surely one of the worst. Synthesized equipment is extremely valuable for EFP, touring groups, lecturers and others who frequently travel from location to location, especially when they have control over all wireless systems at the site. Synthesized equipment also works well as spare or “floating” systems, since they can be set to the same frequency as equipment removed for maintenance or being temporarily used elsewhere. With proper frequency coordina- tion, synthesized systems are invaluable for temporarily aug- menting an existing system for a special event. There are also a number of other applications where frequency agility is highly useful. However, the idea of using agility to permit just showing up and picking some “good” frequencies is often more illusion than reality.

30 Interference and Frequency Coordination

INTERFERENCE “Interference” is often blamed by inexperienced users as the Wireless mic systems generally operate in several bands from cause of noise in a wireless system, even though there are several 150 MHz to 216 MHz, which includes the VHF TV channels 7 other common causes of poor signal to noise ratio that com- through 13, or in the 470 MHz to 806 MHz UHF band (TV monly occur. RFI (radio frequency interference) is a nebulous channels 14 through 69). TV channels 60 to 69 (746 to 806 process at best. With respect to wireless mic systems, interfer- MHz) are being re-allocated, as of the date of this writing, for ence is generally defined as an undesired RF signal which causes other applications. In addition, the band from 470 to about 516 noise or distortion. It can also cause limited operating range and MHz is also being re-allocated for public safety applications. drop outs. Interference can result from external RF signal sources The demand for more spectrum usage is increasing while the such as television station broadcasts, or it can be generated available spectrum for wireless microphones is decreasing. within a wireless system itself. Interference is also generated by Above the TV band is another part of the UHF spectrum from operating multiple systems in the same location. To further 902 to 928 MHz. This upper UHF band is a “general purpose” complicate matters, interference can also result from some band being used by a multitude of different applications ranging combination of all of these sources. from garage door openers and , to home-use Interference in a single channel wireless system normally results cordless . Generally speaking, the 902 to 928 MHz from an external RF signal or RF noise near the receiver. This band is not a good choice for wireless microphone systems, type of RFI usually results from a signal on the carrier frequency especially for professional use in traveling applications. Interfer- of the system, or even on an IF frequency. In multi-channel ence is virtually guaranteed in this band. wireless systems, radio frequency interference is a much more Since multi-channel wireless mic systems often utilize inactive complex matter, since the wireless systems themselves can TV channels, one of your first considerations in operating a generate RF interference within the overall system. multi-channel system in a particular area often involves identify- Multi channel wireless systems always require higher perfor- ing the local TV stations. If you try to operate the wireless mic mance components than simple one or two channel systems for system on the same frequency as the local TV transmission, there the following reasons: is little hope that your battery powered transmitter signal will 1. Interference from external sources is a problem for any have a chance of overcoming a local TV station signal (which wireless mic system, whether a single or a multi-channel may be powered by Hoover Dam). As DTV stations come on configuration. In a multiple receiver system there are many the air, and analog telecasts remain active, the available spectrum more possibilities for external RFI. decreases dramatically. 2. In addition to external RFI problems there are “in system” There are also numerous business radio services that share the RFI problems that are generated by the multiple receivers non-broadcast VHF spectrum fairly close in frequency to the and transmitters themselves. These “in system” RFI wireless microphone allocations. RFI from these sources is problems are usually more numerous and harder to cure than usually rare, involving some sort of intermodulation problem, the external RFI problems. rather than from a direct signal on one of the wireless frequen- cies. Other sources of external direct signals can come from two- 3. Furthermore, external sources can combine with the normal way , leaky cabling from CCTV systems, temporarily RF signals in the systems to create additional problems. installed (rental) wireless systems, wireless intercom systems, It is possible to avoid a number of problems by spacing the and numerous other radio devices. wireless frequencies very far apart; however this also restricts the number of systems that are usable in any one location. If the Sound user wants a large number of channels in one location, then some Picture Carrier Carrier of the channels are going to be placed relatively close together. This will very quickly “separate the sheep from the goats,” with respect to the individual wireless system design. EXTERNAL SOURCES OF RFI Wireless mic systems operate within specific frequency bands allocated by the FCC (Federal Communications Commission). Everybody (and their brother’s dog) wants more spectrum space to operate all kinds of RF devices at whatever power levels they might need to make their particular devices work. The applica- tions include wireless mics, intercom, IFB, , communications, video signals, transmissions, and so on. The simple fact is that there is not as much spectrum space available as there are demands on using it. So, what we are left with is “shared spectrum space,” where wireless mic systems utilize the same frequency bands as other “more important” users.

31 Wireless Microphone Systems

In addition to direct signals from external radio devices, there are “3rd order IM” can cause real problems that can not always be also numerous sources of RFI possible from what is called “man prevented by highly selective receiver front-ends. In the case of made noise.” This is generally broad band RF noise generated 3rd order IM, it is possible for the interfering signals to be by a number of different types of devices including switching simultaneously close together and close to the receiver’s operat- power supplies, , computer peripherals, computerized ing frequency. In this case, the interfering frequencies will pass telephone systems, processing equipment and a right through the front-end filters in the receiver and generate the broad assortment of electrical power equipment. Locating the IM signal in the first mixer. sources of RFI from these types of sources is usually a matter of 3rd order IM can occur from the mixing of three signals, or from turning off other devices one at a time and locating the culprit the mixing of a signal and a second harmonic of another signal. through a process of elimination. This primarily occurs in two places in wireless systems; at the Doing a “sound check” for the wireless system is just as neces- first mixer in the receiver and between the transmitters them- sary as checking out a sound system itself. TV stations normally selves. If two transmitters are within several feet of each other, operate with continuous carriers 24 hours a day, so if RFI is the transmitter output stages can mix the two signals together going to be a problem generated by the local TV transmissions, it with very interesting results. will usually be constant. Business radio services, however, usually operate within normal business hours of 8PM to 5PM, so 3rd ORDER INTERMODULATION evening hours are generally free of business radio interference. ANY NON-LINEAR Other radio signals (and there are lots of them) in the area may DEVICE operate at any time, so you simply cannot predict when they 184 MHz (184 x 2 + 185) 553 MHz might generate an interfering RF signal. 185 MHz (184 x 2 - 185) 183 MHz The best approach is to select clear TV channels, complete a frequency coordination plan for the systems and use only very In this 3rd order IM example, the second harmonic of 184 MHz high selectivity receivers that also provide high IM and image (368 MHz) mixes with 185 MHz, producing a signal at exactly rejection. If you do not understand how to rate the selectivity or 183 MHz. Obviously this is going to create a problem with the IM rejection capability of a particular receiver, call the manufac- system at 183 MHz, since the 183 receiver will respond to this turer. If they can’t give you a clear explanation, you should look IM signal just as well as it will to its own transmitter. Radio elsewhere, because this is one of the most fundamental aspects of signals will combine to produce IM signals through second, any wireless mic receiver. Marketing “hype” in advertising is third, fourth, fifth, sixth, and even seventh order combinations. one thing, but reliable RF performance is another. Multi-channel wireless systems work reliably, however, after INTERMODULATION much time is spent in frequency coordination and optimum on- All active devices, such as transistors, are non-linear. When two site antenna placements are made. The reliability factor im- or more signals are present at any level in a non-linear electronic proves dramatically if you use only high quality receivers device, a phenomenon called “intermodulation” occurs. In an designed for multi-channel environments. The performance audio amplifier this would be called “intermodulation distortion,” specs on a receiver can be a bit nebulous, but among the most or “IM distortion.” For example, if two signals are present at the important specs for multi-channel capability are selectivity and same point in a circuit component, a sum and a difference signal third order intercept. will be produced. This is called 2nd order IM, since there are Selectivity is a specification that indicates the bandwidth of the receiver front-end filters and the IF filter stage. An excellent 2nd ORDER INTERMODULATION receiver front-end will exhibit over 20dB of suppression of RF 89 MHz ANY SUM 185 MHz signals at +/- 7MHz away from the carrier. IF filter performance NON-LINEAR is generally rated as a specified bandwidth between the half- 96 MHz DEVICE DIFF 7 MHz power (-3dB) points either side of the IF frequency. The highest selectivity, fixed frequency receivers available have IF band- widths of less than 50KHz and 60 dB of rejection at a bandwidth two frequencies involved, and first harmonic of each frequency is of 90 KHz. High performance wideband systems utilize wide the frequency itself. deviation such as +/-75kHz, which requires that the IF filter In this example of 2nd order IM the frequencies (89 and 96 bandwidth be several hundred kHz wide to avoid distortion. MHz) fall within the commercial FM radio band. Even though “3rd order intercept” refers to the input level of interfering the commercial FM radio band is generally very full, and the signals required to produce distortion (3rd order IM) of the same transmitters often radiate up to 50 Kilowatts, these frequencies magnitude as the interfering signals inside the receiver. A fairly are so far below the frequency of a wireless receiver, that the good receiver will have a 3rd order intercept spec of somewhere receiver front-end and IF filters can easily reject them. So, the around -15dBm. The best receivers available will have a 3rd theoretical signal of 185 MHz in this example is never actually order intercept spec of +10dBm or higher. generated in the receiver. 2nd order IM from two external sources like this rarely creates a problem in the receiver except in two unusual situations which are discussed later in this chapter in the section entitled “RFI IN SYSTEMS.”

32 Interference and Frequency Coordination

RFI IN TRANSMITTERS Obviously if two wireless systems are too close together in If either of these symptoms occur between transmitters, your frequency (less than 400 KHz) they can generate audible solution is to keep the transmitters farther apart or change interference in the receivers, or there will be a large reduction in frequencies on one (or both) of the transmitters. Moving the the operating range of one or both systems. However, even transmitter to the other side of a person’s body may solve the transmitters that are widely separated in frequency can produce problem. Just remember to check this out, or a passionate interference. Crystal controlled, non-synthesized transmitters embrace between two performers on stage may sound like produce not only the desired carrier but also a number of “spurs” “Robbie the Robot meets R2D2.” (spurious emissions) at lower power levels above and below the RFI IN RECEIVERS carrier frequency. For a transmitter at 180.000 MHz there will typically be spurs at 15 MHz intervals on both sides of the All wireless mic receivers operate through a process called carrier, namely at 135, 150, 165, 195, 210, and 225 MHz. “super-heterodyning.” A local oscillator inside the receiver Interference will occur in a receiver operating on or close to one generates a fairly strong signal that is mixed (heterodyned) with of these spurious frequencies. the incoming RF signal in the mixer stage of the receiver. The result is a “sum” and a “difference” signal. The “difference” 180 signal (called the Intermediate Frequency or “IF” signal) is then heavily filtered and converted into an audio signal (demodu- lated). This process is used to lower the radio signal frequency to make filtering and much easier. For instance, a 135 225 carrier at 194.7 MHz would be mixed with a local oscillator at 165 195 184.000 MHz to produce a standard IF frequency of 10.7 MHz.

150 210 SINGLE CONVERSION SUPERHET RECEIVER

10.7 RF FRONT MHz MIXER IF AMP DETECTOR 194.700 END IF MHz FREQ 15 mHz 15 mHz 15 mHz 15 mHz 15 mHz 15 mHz OSC AUDIO AUDIO 184.000 IF EXPANDER CARRIER MHz FILTERS AMP You can test for transmitter spurs by turning all the receivers on and then turning on one transmitter at a time. If two receivers The oscillator in a superhet receiver can radiate energy outside of come on at the same time, turn off the receiver that matches the the receiver housing, usually through the antenna port. This transmitter and see if the other receiver remains on. If it does, radiated energy can easily enter another receiver located next to you’ve probably got a spur from that transmitter. If the problem it, injecting the signal into the adjacent receiver. When this goes away when the transmitter is moved farther away, and the happens, the adjacent receiver can respond to signals from the transmitter will always be used at that distance (or farther), you first receiver. In other words, one receiver can generate interfer- will probably be OK. High quality transmitters have output ence for another receiver sitting next to it, even though neither filtering that will reduce spurs, but it is difficult to eliminate them transmitter is turned on. A 184.000 MHz receiver mounted in entirely. Proper frequency coordination is the best solution. the same rack with a 194.700 MHz receiver can easily pick up the local oscillator from the first receiver as well as it will its own Transmitters can also produce interference if two transmitters are transmitter. within several feet of each other. The RF signals can combine in a number of interesting ways, some of which can cause you real Through careful design, LO crosstalk like this can be minimized problems. Third order IM is one common problem. The other or eliminated altogether. The FCC regulates how much LO problem is overload of the output stage in one or both transmit- radiation is allowed, but the allowable tolerances are far above ters. the low levels that can create problems in multi-channel wireless systems. A well designed front-end in a receiver is instrumental The symptom of 3rd order IM is interference in a receiver that is in minimizing LO radiation from the antenna port. A simple test not on either of the two transmitter frequencies. For instance, of placing the receivers next to each other and observing the transmitters on 183 and 184 MHz can generate interference in a squelch indicators (usually labeled “RF”) will usually tell you if receiver on 182 MHz, if the transmitters are within a few feet of there is a problem with LO crosstalk. each other. Since 3rd order IM is discussed in more detail in the section dealing with receiver problems, just remember that it can happen between transmitters as well as in receivers. Proper frequency coordination is always the best solution. Another problem related to transmitters is caused by RF energy from one transmitter antenna coupling into the other transmitter antenna and causing output stage instability or overload. The symptom here would be one or both matching receivers squelch- ing or producing very nasty noises.

33 Wireless Microphone Systems

The local oscillator in a receiver can also generate other spurious An example of this more subtle problem is illustrated below. RF signals that are not as obvious as in the previous example. Most manufacturers of wireless receivers have chosen receiver operating frequencies that are compatible for multiple installa- tions. Using receivers made by several different manufacturers in the same installation, however, may lead to some unpleasant TX surprises and lots of finger pointing. 183.000 You can test for LO crosstalk by hooking up the receivers exactly as they will be used (rack, audio cables, antennas, grounds, etc). Turn all the receivers on with all the transmitters off. If one or more of the receivers is indicating that it is receiving a signal, turn off all the other receivers. If the signal disappears, you RX 183.000 RX 185.000 probably have crosstalk. Then, try turning on the other receivers 172.300 one at a time to locate the culprit. The simplest solution is to change the frequency of either the offending or the offended RX 183 L.O. receiver or re-locate one of them. You will then have to try the same test again, of course. AUDIO OUT TX 183 RFI IN SYSTEMS (Rx/Tx combinations) AUDIO SIGNAL Second order IM is generally easy for a receiver to reject since Receiver 183 has a local oscillator (LO) frequency of 172.300 the signals required to generate a problem must be far from the MHz (183.000 - 172.300 = 10.7 MHz). If this local oscillator operating frequency of the receiver and can be easily rejected by leaks into receiver 185 (which it can) there would appear to be the front-end filters. Remember that the frequency of a second no problem since any competent receiver can reject a signal 12.7 order IM signal is produced by the simple sum or difference of MHz off frequency. But, a problem suddenly appears anyway the frequencies of two other signals. For instance, to generate an when transmitter 183 is also on. Transmitter 183’s carrier and interfering signal at 185 MHz would require two signals either receiver 183’s local oscillator combine in receiver 185’s mixer to 185 MHz apart or two signals that would sum together to produce a 10.7 MHz signal (183.000 -172.300 = 10.7 MHz). produce 185 MHz. Mathematically, at least one of the signals The problem is that both receivers will respond to the same must be at least 92.5 MHz from the receiver’s operating fre- transmitter, even though they are on different frequencies. quency (half the carrier frequency), which is easy for even standard front-end filtering to reject. The reverse can also occur: receiver 185’s local oscillator will be at 174.300 MHz and can combine with transmitter 185’s carrier Second order IM can also be a problem if you have two transmit- to produce a 10.7 MHz IF signal in receiver 183. In a well ters that are separated in frequency by the IF frequency of the designed receiver, the local oscillator leakage will be minimal receivers (commonly 10.7 MHz). For instance, if you have and this problem will only surface when the corresponding transmitters at 185 MHz and 195.7 MHz, the difference is 10.7 transmitter signal is strong. If the receiver has a high selectivity MHz. This difference signal may interfere with any receiver with front-end, this problem will be further reduced. Since the two a 10.7 MHz IF operating within 5 to 10 MHz of these frequen- interfering signals are spaced apart by the IF frequency (10.7 cies. For instance, a receiver at 193 MHz is only 2.7 MHz away MHz in this example), at least one of the signals will be attenu- from 195.7 MHz and only 8 MHz away from 185 MHz. A ated by a high selectivity front-end. standard front-end may have only a few decibels of rejection for signals this close in and the signals will pass through the front- To test for this problem, turn on all the receivers, positioned and end to the mixer stage, which will generate a 10.7 MHz signal connected exactly as they will be used, and turn on the transmit- from these signals. Note again that it makes no difference what ters one at a time. The transmitters should be about 10 or 12 feet the receiver frequency is. As long as it is close to or between 185 away from the receiver antennas. The matching receiver will and 195.7 MHz, it will have a problem with these two transmit- unsquelch (RF lamp comes on), of course, but watch for other ters. Obviously, you don’t want to have transmitters spaced at receivers also unsquelching. If one or more other receivers the IF frequency of any receiver in the system. Receivers with a unsquelch, turn off the receiver that matches the transmitter. If highly selective front-end and high level mixers will help prevent the other receivers then squelch properly when the matching this problem. Again, proper frequency coordination will alleviate receiver is turned off, you have this problem. this problem. You can try moving the transmitter farther away. At the actual A subtle problem that can also occur in any multi-channel operating distances this problem may disappear. If it remains a wireless system, no matter what operating frequencies are problem at 30 feet or more, you may need to make major shifts chosen, is similar to receiver crosstalk discussed earlier. Assume in system frequencies. Small shifts will not solve this problem. that two wireless systems are operating at 183.000 and 185.000 If one receiver and matching transmitter cause all the problems, it MHz (you can choose any pair of frequencies as long as they are is probably excessive local oscillator radiation from that receiver. within 10 MHz or so of each other). Assume that both receivers have IF frequencies of 10.7 MHz (the most commonly used). We’ll designate the parts of these systems with 183 and 185 to keep them straight.

34 Interference and Frequency Coordination

You can simply try replacing it with a different receiver and SOLUTIONS TO RFI PROBLEMS transmitter. Since most multi-channel wireless mic systems utilize inactive Some ways to reduce this problem are: television channels, the geographic location must be determined 1) use antenna combiners that isolate the receiver at the outset. TV stations in the vicinity must be taken into antennas from each other account first, followed by at least several “runs” of a computer 2) use receivers with low local oscillator radiation program to determine what sort of IM problems might occur 3) use receivers with highly selective front-ends. within a particular group of possible frequencies. The better 4) separate the receivers by several feet or more computer programs include automatic selection and testing of frequencies within available bands, followed by a report of the Another basic problem that often occurs in multi-channel results. In most cases, choices have to be made between the wireless systems is a matter of 3rd order combining of the frequency groupings with the fewest overall IM problems, and carriers. To illustrate this problem, assume that you have the practicality of delivering what is available on time, given the wireless systems on 183.000, 184.000, and 185.000 MHz. The particular sound system application. Even if you have some sort RF front-ends of the receivers will provide only a small amount of computer program to do the calculations, there are still a of attenuation since there is only a 2 MHz spread between the number of choices and a few “judgment calls” that will need to

183 RX be made. In addition, the particular filtering performance of each 183 TX system must be taken into account. (184x2=368) Any wireless mic manufacturer who is truly involved with high- end wireless systems will have a computerized frequency (368-185=183) 184 coordination program available. In most cases, these programs are not available as “public domain” software since the param- 185 eters that make them valid change as the wireless systems are re- 184 TX 184 RX designed. There are thousands of calculations that must be made in order to select a group of frequencies that are useable for any particular location. This is why computers must be used. There is no way you could do all the calculations necessary by hand on a timely basis. Always contact someone with experience with 185 TX 185 RX this process and work out a frequency coordination scheme before you get involved with a multi-channel wireless system. It is always best to implement a multi-channel wireless system using only identical systems from the same manufacturer on coordinated frequencies. Every wireless manufacturer has taken frequencies (1.2%). All these signals will pass through the front their own path in designing wireless systems. There are numer- end filters of all three receivers. ous choices made by design engineers to determine the oscillator Assume that the signal from transmitter 184 produces a 2nd fundamental frequencies, the multipliers to be used, and the IF harmonic (2 x 184 MHz) in the mixer stage of receiver 183. The frequencies in any particular design. Trying to mix different signal from transmitter 185 (which also gets into receiver 183) is brands and models in a multi-channel system is just asking for subtracted from the 2nd harmonic of 184, and the resulting signal problems. Putting 3 or 4 channels into a church in Buford’s is just as valid as the signal from the 183 transmitter. Point, Idaho is one thing. Trying to place 8, 10 or more channels on the road touring the US is an entirely different matter. (184.000 x 2) - 185.000 = 183.000 MHZ Synthesized, frequency selectable wireless systems can facilitate Obviously, you can prevent this problem by changing any one of the frequency changes needed to achieve a compatible grouping the three frequencies. In a large multi-channel wireless installa- of frequencies in a particular situation, but there is also a “dark tion, things aren’t quite so easy to fix, since the possible combi- side” to this agility. nations become mind boggling. In a 24 channel system there are 552 third order IM products. Changing one frequency to get rid Synthesized transmitters typically generate fewer and lower of one interference problem can create 5 new ones. spurious emissions, which eases the complexity of coordination a bit, but the problems of crosstalk and intermodulation just If you also include the case of three transmitters getting into a discussed will still exist. Imagine a crowded news gathering receiver, the calculations become even more tedious and a event with a dozen or so wireless systems operating. Obviously, computer program becomes absolutely necessary. To make there is no time to get everyone together and work out a useable matters even more unpredictable, the wireless system carriers can frequency mix. Even if you could, a late arriving crew may also mix with signals from external sources, or the external bring in yet another frequency and create the need to do it all signals alone can mix to generate the same sort of IM problems. over again. If everyone using a frequency selectable system It is virtually impossible to predict all the combinations that starts switching around at random to find a clear channel, the could occur in any one locale. The best advice is to use only odds that you end up with a compatible mix in a reasonably short receivers offering very high IM rejection and very high selectiv- length of time are slim to none. Frequency selectability in this ity. A computerized frequency coordination program is a must scenario may well make things worse. for any medium to large multi-channel wireless system of 6 to 24 channels.

35 Wireless Microphone Systems

COMPUTER INTERFACE With the advent of microprocessor control, a powerful tool is external signal is fairly weak, the transmitter signal will bury it now available in high-end receivers offered by Lectrosonics to and the system will still operate OK. A strong external signal, assist in identifying RFI and find clear operating spectrum. The however, can create noise even when the receiver’s own transmit- software supplied with the receiver provides a graphical display ter is on. of all internal settings and status, and enables downloading and 2) CHECKING FOR TX SPURS AND 2ND ORDER IM uploading frequency groups to and from the receiver, as well as adjustment of a variety of operating modes. Utilizing an RS-232 Turn on all the receivers. Then, turn on the transmitters one at a compatible PC interface for the Windows® operating system, the time. As each transmitter is turned on, its corresponding receiver receiver can be also used to perform a “site scan” when setting will unsquelch (RF lamp comes on). Look to see if any other up a wireless system in a new location. receivers also unsquelch at the same time. If one or more others does come on, turn off the receiver that matches the transmitter and see if the other receivers remain unsquelched. If the squelch remains open, then you probably have a transmitter spur getting into the other receivers. If they squelch and shut down normally when the matching receiver is turned off, you may have a 2nd order IM problem as was discussed in the text earlier in this section entitled “RFI IN SYSTEMS.” Refer back to that text for more information on this type of RFI problem. 3) CHECKING FOR 3RD ORDER IM Turn on all receivers and all transmitters. Place the transmitters at the closest distance from the receiver antenna/s that they will be in during actual use. One at a time, turn the transmitters off, and then back on again. Do this at least 5 or 6 times for each transmitter as you move around. Moving around with the The lower section of the display provides a graphical, scanning transmitters as you turn them off and on will insure that you spectrum analyzer for conducting site surveys. During scanning, don’t have one located in a “null” where the RF signal is very the receiver is tuned in steps across its tuning range and markers weak at a receiver where you would normally have a problem. are place on the screen to indicate the frequency and signal Check to see if the corresponding receiver squelches (no RF strength of signals found. The advantage of using the receiver lamp) as its transmitter is turned off. If it doesn’t, you will need rather than separate test equipment for scanning is that the to determine what combination of transmitters creates the RFI receiver will display not only external signals in the vicinity, but signal. also RF signals produced by intermodulation that might occur Through what can sometimes be a rather lengthy process of inside the receiver. The result is a thorough analysis of the site, elimination, you can narrow down the problem and identify and a clear picture of the usable spectrum available. which particular combination of transmitters generates the TESTING FOR COMPATIBILITY offending IM signal. The solution to this type of IM problem In the “real world,” a touring sound company can rarely enjoy often involves changing frequencies of one or more of the the luxury of purchasing or renting a new collection of wireless systems. Sometimes, moving the receiver antennas farther away mic systems each time they move to a new location and imple- from the transmitters can reduce the IM problem. ment a multi-channel wireless system. It would be nice to go 4) FINAL SYSTEM CHECK OUT through all the steps and check out procedures previously Turn everything on. Listen to the output of each system one at a mentioned for each job, but the real world just isn’t that friendly. time. The idea in this step is to check for bad connections, The following procedure is useful in determining the basic transmitter gain adjustments and receiver output levels. compatibility of a multi-channel wireless system, both for touring sound companies and in fixed sound applications. 1) CHECKING FOR RECEIVER INTERACTION Turn on all receivers and place them in the same relative position they will be in in actual use. Leave the transmitters off. Check to see if any of the squelch indicators (usually labeled “RF”) on any of the receivers light up. If any receiver squelch opens, turn off the other receivers one at a time to locate the receiver generating the RFI signal. By repositioning the offending receiver, you may be able to alleviate the RFI problem which, in this case, could be the result of LO crosstalk. If repositioning the receivers does not change the problem, you may have an external RF signal mixing with one of the receiver oscillators. In this case, turn on the transmitter for the receiver that the squelch opens on, and see if the audio sounds OK. If the

36 Interference and Frequency Coordination

FREQUENCY COORDINATION Interference is never considered acceptable in wireless micro- This is not intended to downplay the value of synthesized phone systems, yet it is not all that uncommon. When interfer- wireless systems. The flexibility of being able to retune a system ence does occur, it is embarrassing, disruptive and annoying. For to replace a failed or unavailable system, the ability to go to other audio professionals responsible for wireless systems, it also cities having different TV channels, increased equipment usually means costly trouble. Yet, most problems with wireless utilization and many other benefits make such synthesized interference are preventable. Only in a relatively small percent- equipment deservedly popular. The point being made is that age of instances is the interference truly unavoidable. The fact is there are many situations where use of synthesized gear, if not that most interference is caused by other wireless systems, properly controlled, can cause as many problems as it solves. A broadcast transmitters, handi-talkies and other known or control- crowded news gathering event is prime example. lable sources. With an early start and an orderly approach, simple measures can be taken that will greatly reduce the PROGRAMS AND TOOLS chances of interference. As was discussed above, computer programs are more or less Frequency coordination is a procedure for ensuring, in advance, essential in performing frequency coordination. Virtually all that predictable causes of harmful interference are avoided. wireless manufacturers use such programs at least occasionally. Whether or not each of these sources is a potential cause of Often, however, manufacturers do not make them available to interference can be analyzed mathematically and, if so, appropri- their dealers and customers, since the data required for valid ate preventative measures taken. The analysis can, and often calculations is changing constantly, and specialized training is does, begin before the wireless equipment is even purchased. If required to correctly analyze the results of the calculations. If a supplied with adequate information about the location of the site, suitable program is not available to the user, how then is fre- about other wireless systems at the site, and details on other radio quency coordination accomplished? equipment present at the site, the wireless dealer or manufacturer There are several choices. Many wireless manufacturers will can select optimum operating frequencies for the new equipment. perform frequency coordination for new systems being pur- chased, usually including existing wireless equipment from other SYNTHESIZED WIRELESS SYSTEMS wireless manufactures. Many dealers offer the same service. If At first glance, it might appear that use of synthesized wireless more than one brand of equipment is being used, perhaps one equipment would eliminate the need for frequency coordination manufacturer will provide this service even if the other will not. and analysis. In a few instances, such as for a small number of There is a significant element of risk, however, with using a systems that are all under control of one or two team members, program from one manufacturer with equipment from another. this will be true. In these cases, the team will usually be able to Maintaining accurate data from several manufacturers to include turn on all of the equipment and find interference-free frequen- all of the subtle sources of potential RFI in a single database is cies by trial and error. Even this has some pitfalls, however, virtually impossible. Since a particular program might or might since it might be difficult to fully recreate the complete RF not be usable for other brands of wireless equipment, its suitabil- environment during testing. In addition, intermodulation levels ity should be verified with the technical staff of the manufacturer. depend greatly upon the actual signal levels present, and moving Otherwise, there is a good chance that the results obtained will be just one transmitter a few feet closer to a receive antenna might seriously in error. introduce a number of additional problems. A few dealers and wireless service centers offer frequency In more typical situations, the use of synthesis not only does not coordination and related services on a fee basis. This may be a eliminate the need for coordination, it can make the task more good choice if the equipment manufacturer and dealer does not difficult. In many broadcast and production environments provide the necessary support. This type of support might also several different production groups might be operating wireless be available at night and on weekends, an option not generally systems in close proximity to each other. If one group takes it available from manufacturers. It is suggested that the wireless upon themselves to change the frequency of a system, it is not manufacturer or dealer be contacted to see if they can provide unlikely that the change will suddenly introduce problems into such services, or can make a referral to someone else qualified to the systems in other wireless systems, or on adjacent stages. If do frequency coordination. the second group then changes the frequency on one or more of their systems, a third group or the first group might well now Third party “intermod” programs are available from several experience problems. Without some control and discipline, it is sources, both commercially and as shareware. The commercial easy to see how the situation can escalate out of control, wasting programs tend to be relatively expensive. The quality of the large amounts of expensive production time. shareware programs is extremely variable, often making invalid assumptions about equipment characteristics. In both cases, A typical compromise is to find a small number of additional these programs often do not test for certain significant types of trouble-free frequencies for each of the groups. If a change is wireless interference. At the same time, they might identify then desired, the new frequency is selected from the supplemen- “problems” that do not really exist, such as very high order tal list, providing a reasonable degree of assurance that other intermodulation products. Use of these programs is best left to groups will not experience new problems. The difficulty is, of persons familiar with both wireless systems and the technology course, that finding enough extra good frequencies can be involved. extremely tough and the task will certainly require much additional effort. Most facilities eventually adopt some type of centralized control in order to minimize conflicts.

37 Wireless Microphone Systems

It should also be observed that an appropriate level of support sometimes omitted from published listings or appear only in from the manufacturer, dealer or service supplier could, as a small separate sections of a publication. practical matter, prove to be a necessity. That is, if situations that require frequency coordination are relatively common, and SITE SURVEYS needed support is not readily available, it might be wise to Sometimes the question of doing a site survey is raised. The idea consider another supplier. A wireless system that doesn’t work is that suitable monitoring instruments be brought to the location properly, at any price, is never a bargain. Faulty frequency and “all of the signals” that could interfere with the wireless will selection can be every bit as damaging as an equipment failure. be found. While the sentiment is understandable, especially if In addition to an appropriate computer program, other data is there have been recent interference problems, this approach is not required in order to perform an accurate coordination. Two very reliable. The problem is that many potential interference particular pieces of information are especially important; the sources only operate intermittently; two-way radio being a active TV channels in the area of use, and the frequencies and common example. Readily available monitoring instruments are types of all other wireless systems and RF equipment that will be not suitable to capture and record information on very brief at the location. In a majority of applications, wireless-to-wireless random transmissions and human monitoring is highly error interference is the most common form of troublesome interfer- prone over any extended period of time. ence. It is also one of the easiest to avoid. Simply providing the Other difficulties include the inability of reasonably priced computer program with the frequencies and types (or characteris- instruments to determine and record the exact frequency of a tics) of all wireless at the location will allow it to select interfer- source; an essential part of using the data in a coordination ence-free frequencies 98% of the time. program. An error of only 10 to 25 kHz can make the difference The importance of including all equipment to be used is difficult as to whether or not a specific source is likely to be an actual to overstate. Just one extra wireless transmitter at the location problem or just a near miss. Also of concern is that the wider the can, and often has, resulted in serious problems with a consider- frequency range to be monitored, the longer it will take. This is a able number of other wireless systems. Obviously, this also serious limitation if more than a 3 or 4 MHz bandwidth needs to applies to systems added at the last minute, or any changes to the be checked. Finally, sources at the site itself are most likely to be coordinated frequency list (including resetting a synthesized a problem, and they might not ever be operated until the event system). Basic information, such as manufacturer, model takes place. Good examples are portable communicators for number and frequency about other wireless systems in use is also security personnel and special tactical two-way frequencies used required. This includes such items as wireless intercom systems, by police details. wireless IFB equipment and wireless musical interments. The most serious limitation of all is that much of the interference Without this information, the programs cannot prevent the experienced by wireless systems is due to intermodulation that possibility of certain types of interference to these other systems. occurs inside of the wireless receivers. In this case, there not be Basic information on other RF sources at the location is also an actual RF signal at the proposed wireless frequency that could needed. This includes portable two-way communicators, CATV be detected by a separate instrument. Thus, the idea of “search- or MATV systems, data and transmitters and similar ing” for good frequencies with an instrument is more or less items. Transmitters on frequencies below 108 MHz or above unworkable. Considering these shortcomings and the typically 1000 MHz can almost always be safely ignored unless they are high costs for site surveys, it is necessary to question both their extremely powerful. Gathering this data is not always easy, accuracy and whether or not they are cost effective. especially when the site is in a distant city and the inquiries must Despite the above, there are a few situations where a simplified be done by telephone, e-mail or . However, obtaining on-site survey can be of value. One example is when the accurate information is the only real way of avoiding nasty wireless systems will be used at a considerable elevation, such as surprises a half-hour before a live performance. on the top floors of a tall building or on a hilltop. In this case, Channel numbers for local TV channels are also needed. Often, strong signals from TV stations or other transmitters at surpris- manufacturers and dealers will have this information available ingly great distances are sometimes encountered. Another from reference documents and only the city of operation will example is when wireless systems will be used near or need to be specified. It is almost impossible for the references to government installations or high power radar sites. Special be totally up to date, however, so an inquiry as to whether any purpose government equipment sometimes uses frequencies that new TV channels have gone on the air in the city within the last can interfere with wireless microphone equipment, but this is month or so is advisable. This is mostly a problem at UHF rarely public knowledge. frequencies since relatively few VHF stations are still being High power radar systems such as that used by the military and constructed. The advent of DTV has worsened this situation. the aircraft traffic control network can also be troublesome, Unfortunately, accurate and up to date TV information is not especially near airports and remote radar sites. In most cases, the easy to obtain and can be relatively expensive, so a local contact interference is not due to frequency conflicts, but rather the is usually the best source. circuit effects of very high power signals. This can Only high band VHF TV channels numbers are needed if only be a difficult situation to resolve, making early warning of the VHF wireless systems are being used. If UHF wireless systems problem very important. Another somewhat similar situation or a combination of VHF and UHF systems are to be used, both occurs in and around heavy industrial areas. Here, high power UHF and high band VHF TV channels might be required. Do electrical equipment can cause various types of interference not leave out public and educational TV stations, which are problems, especially at VHF frequencies. A relatively quick

38 Interference and Frequency Coordination survey will usually be adequate, but care must be exercised to SOME GENERAL SUGGESTIONS ensure that testing is performed at the appropriate time of day Here, in no particular order, are a few suggestions regarding and day of the week. frequency coordination and management of wireless systems: The bottom line is that site surveys with test equipment can be • Always ask for frequency coordination when purchasing new worthwhile in certain circumstances, but lengthy and elaborate systems. surveys have little more value than simpler ones. In addition, the limitations of surveys should be kept firmly in mind in order to • Obtain the support of the equipment manufacturer or dealer if avoid placing too much faith in their results. The best approach reasonably possible. is to use the actual wireless receivers that will be used in the • Don’t attempt to go without coordination even if support is venue to conduct a site survey, so that all IM products will be difficult to obtain. included in the results. • For all coordination, provide as much and as detailed informa- INTERNATIONAL COORDINATION tion as possible. The basics of performing coordination are the same in all • Be aware of all wireless equipment brought to the site, even on locations, but there are some practical concerns. One is that the a temporary basis. computer programs normally used in the US might not be valid • Rerun the coordination any time additional RF equipment is to for locations outside of the US and Canada. Many such pro- be used at the site. grams have TV channel data coded directly into the program. • Have a plan for a usable backup if equipment is lost, stolen, This data is invalid for many areas of the world, as TV standards damaged or simply fails. and exact channel frequencies vary significantly from country to • Keep the pertinent data readily available in case last minute country. Wireless equipment technical standards also vary significantly from country to country, so the version of the changes require rechecking the coordination. program available might not provide accurate results when used • Keep TV channel data up to date and close at hand. with wireless equipment intended for use in other countries. • Always keep in mind that just one new RF source can invali- Also keep in mind that wireless mics are still illegal in some date the entire coordination. countries and heavily restricted in others; just because the A final observation is in order. This is that frequency coordina- equipment can be made to work in these areas does not mean tion works extremely well in avoiding problems from frequencies that it will be legal. For example, the eight special VHF “travel- that are known and have been considered by the computer ing” frequencies from 169 to 172 MHz popular in the USA are program. The process does not offer any protection from RF not available in most other countries, even Canada. In most signals that were not evaluated by the program and interference areas, this frequency range is set aside for government communi- from such signals is entirely possible. Sometimes local RF cations and private use is highly discouraged. sources are simply overlooked when the list is compiled or have Obtaining the required information might be more difficult due gone unnoticed for one reason or another. More often, the to language barriers and terminology differences. Despite the problem is due to equipment that has been introduced to the site problems, full and complete information is still essential if after the frequency list was prepared, illustrating once again the trustworthy results are expected. TV channel information is need to be aware of all RF equipment at the operating location. often especially difficult to obtain. It is an unfortunate fact that on rare occasions RF signals appear that simply should not be there. Sources for these signals include special government equipment, unapproved digital and comput- ing devices, defective commercial communications equipment and illegal RF devices of various types. The best protection against problems due to these unexpected interfering signals is to use only high quality wireless equipment and complete the sometimes arduous task of a complete system check out as outlined earlier in this chapter. The best results will be found with receivers that offer overload-resistant RF circuitry, tight RF and IF selectivity, and optimized demodulators and audio filtering. Very often, high quality wireless equipment will operate flawlessly in the face of interference that renders less capable systems useless. For critical applications, an investment in high quality professional wireless always pays handsome dividends over the long term.

39 Wireless Microphone Systems

40 Multi-Channel Wireless Systems MULTI-CHANNEL WIRELESS SYSTEMS

Given the problems of IM and interference discussed in the MECHANICAL ASSEMBLIES section entitled INTERFERENCE and FREQUENCY COORDI- It is generally good practice to keep power cables away from NATION, it should become apparent how complex a multi- signal cables in any installation. If each receiver has its own channel wireless system really is. The receivers must be power cord, it often requires some careful routing of the cabling designed for a multi-channel environment or you can expect in the rack or transit case to avoid AC hum induced in the audio problems from both internal and external RF sources. cables. This is where using low voltage DC powered receivers RF/POWER DISTRIBUTION comes in very handy. If the RF distribution module also includes DC power distribution, you easily can put together very effective From a practical standpoint, multi-channel wireless systems must multi-channel receiver assemblies without creating additional provide some means of “stacking” the receivers together and complexities. sharing common antennas. Simply stacking up a bunch of receivers next to each other with separate antennas can create a OPERATING RANGE number of problems. Superhet receivers radiate RF energy from As a general rule, it is good practice to install the receiver the antenna ports and, in some cases, even radiate RF energy antenna/s so that a transmitter cannot come within less than 25 right through the housings. The radiated energy is normally at feet or so of it during the performance. The closer the transmitter the oscillator fundamental frequency or at a harmonic multiple of is to the receiver antenna, the stronger the RF signal will be and it. When receivers are placed next to each other, they can interact the more prevalent the IM products. Of course, if the transmit- with one another, unless they are isolated by some method. ters are too distant from the receiver, there is also the risk of Distributing the output of a single antenna to two or more dropouts. So, the best approach is to set up a multi-channel receivers is a bit more complicated than it may first appear to be. wireless system so that the transmitter to receiver antenna First of all, as the signal is divided, the individual signal sent to distance falls within a targeted “window.” Generally speaking, a each receiver becomes weaker. This means that an RF amplifier good “window” for system check-out (and even operation) is not is needed in order to keep the signal level at each receiver high over about 100 feet and not less than about 25 feet. The upper enough to provide a usable signal to noise ratio. The RF and lower limits of this “window” will vary, of course, depending amplifier can also amplify unwanted out of band signals unless it upon the performance characteristics of the exact equipment is preceded by some sort of filtering. Lastly, by connecting the being used. receiver antenna terminals together, RF interaction between the This “window” of distance cannot be maintained in what might receivers will occur unless some sort of isolation is provided. be called an over-shoulder “bag system” that consists of several So, an effective multi-channel RF “coupler” should include the receivers and a portable mixer, with the mixer output feeding two following features in the order listed: transmitters that are also located in the bag. The use of a 1) Front-end filtering compact, battery powered multi-coupler in this type of setup is 2) Low noise RF amplification convenient and helpful in dealing with RF interaction between 3) Low loss, high isolation RF splitter the receivers and transmitters. Because of the close proximity of the output transmitters to the receivers, however, extreme care must also be taken in frequency coordination to ensure that the system does not interfere with itself. Generally speaking, a wide SPLITTER/ RF FILTERS RF AMP separation in frequency between the receivers and transmitters is ISOLATOR OUT ANTENNA mandatory, as is the “final system check out” procedure outlined later in this chapter and in the section of this guide entitled AC INTERFERENCE and FREQUENCY COORDINATION. POWER POWER REGULATOR DC SUPPLY OUT FREQUENCY COORDINATION BATT AUTO RESET It should go without saying that any multi-channel system of FUSES more than 4 or 5 channels should be carefully planned. Larger This diagram depicts a single antenna multi-coupler. A diversity systems with 10 or more frequencies can have all kinds of multi-coupler will include a second, discrete RF path for the problems with intermodulation, crosstalk and noise. Computer additional antenna. Compact multi-couplers for field production programs are mandatory in planning the frequencies for larger usually include DC power distribution. Larger multi-couplers, systems, as well as providing a “starting point” before going such as an 8-way diversity type, normally include only RF signal through a check out procedure for any multi-channel system. distribution. Every manufacturer who is a real “player” in the high-end wireless market will have and use a computer program to predict Another valid approach to designing a multi-coupler consists of frequency compatibility in multi-channel systems. Many dealers using a very hefty RF amplifier that has a very high overload and specialized consulting firms are also available to perform threshold (third order intercept of +40dBm or so) without filters. frequency coordination. Since proper planning involves thou- IM products in this type of design will be minimal, but the power sands of calculations and a considerable amount of time, there is consumption of many high level RF amplifiers often precludes sometimes a fee for providing this service. this approach in compact units designed for field production where battery power is the only option.

41 Wireless Microphone Systems

MIXING DIFFERENT BRANDS OF EQUIPMENT A unique tool is available from Lectrosonics for dealing with It is hard enough to come up with usable frequency mixes for long cable runs. The UFM50 filter/amplifier is a compact device larger multi-channel systems using all identical receiver and that includes ceramic resonator front-end filtering and a high transmitter models, but the process becomes much more difficult quality RF amp in an in-line device with jumperable gain settings when various models and brands are used. Computerized to compensate for losses incurred in long coaxial cable runs. frequency coordination programs include calculations that take into account the IF frequencies, oscillator fundamentals and harmonics and several other aspects of the transmitter and receiver designs. By mixing different brands and models together, it is almost impossible to predict all the signal combina- tions that can occur. LONG ANTENNA CABLES The primary problem with long runs of antenna cable is loss of RF signal through the attenuation in the cabling. It is best to The UFM50 can be powered by external DC power at the keep antenna cabling down to a minimum as a general rule. In connector on the housing, or DC power present on the BNC some installations, however, it may be necessary to run cabling to output jack provided by high quality, rack mount multi-couplers. remote locations in order to place the antenna within the proper THE UFM50 AS A MULTI-COUPLER distance “window” mentioned above (see previous paragraph on The UFM50 can also be used to implement a high performance, OPERATING RANGE). Different types of coaxial cable exhibit compact multi-coupler when used with a high quality, passive different amounts of attenuation. Check the specs on the cabling splitter. The input of the UFM50 is connected to the antenna, you intend to use and see how much signal loss would occur. with the output connected to the input of the splitter. By positioning the antenna closer to the transmitter, a stronger High quality passive splitters, such as the one shown here, are RF signal can be picked up at the receiving end of the antenna. specialized devices offered by only a few companies, yet they are The additional signal that is picked up, however, may be lost in readily available through radio communications companies or, of the attenuation in the cabling. There is a point where you lose course, from Lectrosonics. more signal through the cabling than you gain by positioning the antenna closer to the transmitter. For example, RG-8 or RG-213 are two common types of coaxial cables used for UHF frequencies that have a loss of 7 dB per 100 feet. If the antenna is going to be 25 feet from the performer, the PASSIVE RF signal level at the antenna will be at –30 dBm on the average. If UFM50 a ¼ mile (1320 feet) of cable is used between that antenna and a SPLITTER OUT ANTENNA receiver, then the signal level at the receiver will be 93 decibels less or -123 dBm at the receiver. This is much less signal than that required to be even detected, let alone be usable. For comparison, if we don’t use a cable at all but just broadcast from the transmitter to the receiver through the air, the loss is 6 dB for each doubling of distance. Now the signal at a ¼ mile 4-way passive will be 36 dB lower than at 25 feet. The signal at the receiver splitter/combiner will be –66 dBm on the average and the reception will be quite good. To do a ridiculous comparison, a full mile of RG-8 cable will have a loss of 369 decibels and would require 457 trillion trillion Watts of power at the antenna end of the cable to produce enough signal to be usable at the receiver end. This would run a 9 Volt down pretty quickly. ANTENNA BANDWIDTH For shorter runs, coaxial cable can offer some improvement in The frequency bandwidth of the antenna becomes a consider- reception. A 100 foot run of cable, for instance, is better than ation when a single antenna is used to feed an RF distribution through the air. Going through the air, the signal system. The main concern is that the receivers connected to the would be -42 dBm at the receiver and using a cable the signal antenna are on frequencies within the bandwidth of the antenna. would be –37 dBm at the receiver, a 5 dB improvement using the Even if a receiver frequency is close to the edge of the antenna cable. Of course, sometimes cables must be used such as when bandwidth causing some loss of signal (3 to 6dB or so), the slight the receivers are mounted in a metal rack, or are located in a loss of RF signal is usually not a problem. When the wireless control room that will shield the receivers from the antenna. The system is being operated near the limit of its range or severe with purpose of this discussion is to illustrate some of the pitfalls in severe multi-path conditions, however, it is good to gather every long cable runs. bit as much RF signal as possible. Minimal cable loss in this case could make the difference in whether or not the perfor- mance of the system is acceptable.

42 Multi-Channel Wireless Systems

FINAL SYSTEM CHECK OUT The check out procedure outlined in the chapter on INTERFER- Lastly, everything is turned on and the audio output of each ENCE should be a standard routine with any multi-channel system is monitored one at a time to check for bad cables, level system. The process first checks for receiver crosstalk with settings, intermittent connections, etc. This check out procedure receivers on and transmitters off. Then the transmitters are should be followed any time two or more wireless systems are to turned on one at a time to check for 2nd order IM. Next, all be operating simultaneously in the same room. transmitters are turned on and then turned off one at a time to check for 3rd order IM problems.

RECEIVER MULTI-COUPLERS

COMMON TYPES OF MULTI-COUPLERS To facilitate the ever increasing demand for more wireless mic channels in a variety of applications, a number of high perfor- mance multi-couplers are now available to combine multiple receivers into convenient assemblies. Multi-couplers are available in a variety of different configurations to provide RF distribution only, RF and power distrbution, as well as complete systems that provide RF and power distribution and a mechanical assembly to mount the receivers.

UHF Quad Pak RF/Power Multi-coupler This is a specialized type of multi-coupler used in motion picture production for location recording with compact receivers. Two distribution modules are provided for diversity receivers, one module includes power distribution and battery charging Rack mount 8-way diversity RF multi-coupler circuitry, and the other is for RF distribution only. A built-in, For high-end studio and stage applications, a number of compa- high capacity, rechargeable battery pack provides power for up to nies offer antenna multi-couplers for the convenience of sharing 12 hours of operation per charge, or the system can be powered antennas with up to 8 receivers. A high quality multi-coupler by external DC. such as this generally provides only RF signal distribution, and The antenna multi-couplers in the distribution modules offer will normally offer 110 to 240 VAC or DC powering, front-end front-end filtering with a 50 MHz bandwidth, a high overload filtering, a high overload point RF amplifier and highly isolated point RF amplifier and (“strip line”) splitter. RF outputs. The unit pictured here offers ceramic resonator filters with a 50MHz bandwidth, and a transmission line (“strip The distribution modules can be removed for use separately, such line”) isolator/splitter. as in an over shoulder bag system or sound cart. The modules are powered by 12 to 16 VDC.

The main distribution module includes a high quality RF multi-coupler and DC power distribution with discrete auto- reset polyfuses for up to four receivers.

The “slave” module will work Four channel rack mount RF/Power multi-coupler separately as a high quality, For use with a variety of compact VHF and UHF receivers, compact RF multi-coupler for Lectrosonics offers an assortment of UHF and VHF rack mount field production. multi-couplers for compact receivers that includes RF and power distribution as well as a 19” rack mount mechanical assembly.

43 Wireless Microphone Systems

44 Signal to Noise Ratio

SIGNAL TO NOISE RATIO

The RF signal level at the receiver antenna varies wildly as the TRANSMITTER INPUT GAIN transmitter moves around, due mostly to multi-path and overall The transmitter input gain is the single most important adjust- distance from the transmitter to the receiver. The background RF ment on any wireless mic system to insure an optimum signal to noise in the environment also fluctuates. RF noise or interfering noise ratio. The audio signal to noise ratio will never be any signals can produce hiss, buzzing, whistles, whining, etc. better than it is at the transmitter input. If the input signal is Normally, the background noise in the system is low enough that noisy at the transmitter, there is nothing else that can be done it is masked by the audio signal, but when the RF level from the later to restore it to its original quality. The audio level is transmitter dips low enough or the background noise in the adjusted with the gain control on the transmitter, while watching system gets high enough, noise can become audible. There are some sort of level metering on either the transmitter or receiver. several different factors which affect the signal to noise ratio of a wireless system. The most difficult problem with properly adjusting the transmit- ter input gain involves duplicating the user’s voice level in Drop outs or noise can result from problems with any one or advance of the actual performance or use. Obviously, you need more of these factors: some sort of metering in order to correctly set the transmitter 1) Transmitter input gain too low input gain. The metering must indicate the modulation level of 2) Transmitter to receiver distance (operating range) too great the radio signal and also limiting in the transmitter. Metering is generally provided on the receiver, but often times the transmitter 3) Environmental RF noise near the receiver antenna metering is easier to use, since the receiver may not be accessible 4) Multipath phase cancellations at the receiver antenna or visible from the transmitter location during setup. 5) Obstructions in the path between the transmitter and receiver antennas 10dB NOISE

6 Transmitter 5 7 TRANSMITTER GAIN SET Signal 4 8 TOO LOW RESULTING 3 9 IN POOR SIGNAL TO Transmitter 2 10 NOISE RATIO. Signal 1 11 S/N RATIO S/N RATIO

Noise TRANSMITTER GAIN SET Noise 80dB HIGHER FOR USABLE

LEVEL LEVEL SIGNAL TO NOISE RATIO. High Noise, Low Signal Low Noise, High Signal NOISE

6 5 7 4 8

TRANSMITTER TO RECEIVER DISTANCE 3 9

2 10 The transmitter to receiver distance has a major effect on the 1 11 signal to noise ratio of a wireless system. As the transmitter moves farther away from the receiver, the overall signal to noise OTHER CAUSES OF NOISE AND DROPOUTS ratio grows worse as the transmitter signal gets weaker. When the system gets near the limit of its operating range, drop outs Obstructions in the path between the transmitter and receiver will become more frequent and a buildup of steady background antennas can also increase audible background noise. An noise (hiss) may be audible. obstruction in the direct path between the transmitter and receiver antennas causes the same effects as increased operating range (a There is a significant difference in various receiver designs with lower incoming RF signal.) respect to the quieting slope. Quieting slope is a measure of the ability of a receiver to achieve full signal to noise ratio at weak Multi-path phase cancellations at the receiver antenna can make RF levels. A high quality, narrowband receiver will typically the background noise audible briefly. This is a drop out, as is achieve full quieting with only a few microvolts of RF signal. discussed in more detail in the section entitled DIVERSITY Low cost, wideband designs require many times more signal to RECEPTION. When a phase cancellation between direct and achieve full quieting. Of course, there are many factors in the reflected RF signals occurs at the receiver antenna, the desired design of a receiver that affect the quieting capabilities, but all signal from the transmitter may not be strong enough to bury the other factors aside, a narrowband receiver has an inherent background noise. This problem is also referred to as a “noise up.” advantage in that the sharp filtering in a narrowband design Environmental RF noise near the receiver antenna is another gathers less noise from the operating environment. The “operat- cause of poor signal to noise ratio. Digital switching devices, ing environment” includes digital video cameras and recorders power supplies, etc. can radiate broadband RF noise. If a noise where the receiver is often mounted directly on the camera or source like this is located near the receiver antenna, the net effect connected to the recorder input. is that the noise floor is raised by the amount of RF noise. In other words, the signal to noise ratio of the wireless system is lowered by the added RF noise.

45 Wireless Microphone Systems

RECEIVER ANTENNA PLACEMENT FOR A RELIABLE RF LINK Monitoring audio signal levels is fairly simple, however, RF signal levels are much harder to measure and evaluate. Aside from this fact that, they also change constantly. Add to this the fact that there are many more RF signals hitting the receiver antenna than the single signal coming from the transmitter. Many of these additional RF signals are almost impossible to predict. Some receivers offer RF level metering. This is very handy in determining the overall RF signal strength. You will have to conduct a walk test, while viewing the RF level meter. If the RF level dips to a low level when the transmitter is in a particular location, reposition the receiver antenna to a new location at least several feet away from where it was when the drop out occurred. The antenna location that produces the strongest RF signal level is not necessarily the location that will produce the best signal to noise ratio. The reason for this is that in some installations the antenna may be positioned close to an RF noise source (synthe- sizer, switching power supply, computer, etc.) and the additional signal strength indicated by the RF level meter may be composed largely of RF noise. Again, a walk test and close listening to the audio output will reveal which location is best. Even if you can utilize some very expensive RF test equipment to evaluate the environment where the system is located, it will still be impossible to predict RF interference from external signals that could appear later. The best insurance against interference problems is to use only high quality, high selectivity receivers for any critical application.

46 Interpreting Wireless Mic Specifications

INTERPRETING WIRELESS MIC SPECIFICATIONS

Specifications for any product always fall subject to whatever made. The above measurements were all made with an “A” parameters are typically used in the markets where the products weighting filter to approximate the ear’s response to the noise. are sold. The “spec game” is played by every manufacturer. The Most manufacturers will use this filter since it improves the allowable tolerances are not strictly controlled and there are few measurements by 3dB to 6dB. standards, so you generally have to qualify or translate a particu- SINAD is a measurement that approximates the audible back- lar set of specifications before you can make valid comparisons. ground noise heard along with a continuous signal at weak RF It is difficult enough to decipher and compare specifications on levels. SINAD is measured by running the system at full conventional audio equipment, but it gets to be very nebulous deviation with a weak RF signal and measuring the level at the with wireless microphone systems. Add to this the fact that some receiver output which consists of signal + noise + distortion. manufacturers have actually published specifications that are Then a second measurement is made after electronically subtract- wrong, which is an unforgivable marketing crime. ing the audio signal (while the system is still running) and The performance of a wireless microphone system will vary measuring the remaining noise and distortion. The first and dramatically from the test bench to the actual application in the second measurements are then expressed as a ratio. SINAD is field. The results of connecting test equipment directly to a probably the most consistent sensitivity measurement at low receiver and measuring various performance specs will be very levels of RF, since it effectively removes the compandor from the different than when the input signal to the receiver is generated circuit. Since the SINAD measurement is made with the system by a weak radio signal coming from a transmitter several in actual operation at full deviation, it is more realistic than a hundred feet away. It is safe to assume that the published specs simple signal to noise ratio measurement. for a wireless mic system are based on ideal RF conditions and a Signal + Noise + Distortion minimal transmitter to receiver distance. SINAD = ------You should always be skeptical of a spec that does not include the basis for measurement, or of a spec that is missing altogether. Noise + Distortion Anytime a particular spec is hard to interpret or is missing, it is a S/N RATIO is a measurement that approximates the background safe bet that the manufacturer might be trying to disguise the noise heard during pauses in speech when the system is operating poor performance. In a few cases, it could also be that the at a given RF level. It is another valid comparison of sensitivity. manufacturer simply overlooked including the spec in the It is listed as the amount of RF signal required to produce a published literature. We at Lectrosonics would like to say that certain S/N figure, often 50dB. The 50dB S/N ratio is represen- we never forget anything as important as a spec, but let’s face it, tative of a minimum usable sensitivity and corresponds to what a we’re human too. So, if you don’t see something on the pub- non-critical listener would accept. S/N RATIO is determined by lished literature, please call us. We’d be glad to tell you more measuring the system at a given RF signal level at full modula- than you ever wanted to know about what’s missing. tion, with maximum receiver output, then turning off the audio modulation and measuring the remaining noise. This will SENSITIVITY produce the RF signal level required for a given signal to noise Good: 1uV for 20 dB SINAD ratio. This is the sensitivity rating of the receiver based upon Excellent: 0.5 uV for 20 dB SINAD signal to noise ratio. This spec refers to the RF input level at the receiver required to The problem with this method of measurement is that the produce a certain signal to noise ratio. Signal to noise perfor- compandor will make the number twice as good as it really is. mance in a receiver can be measured or rated several ways, but SINAD is really the better method to rate a receiver, but it does the most common methods are “SINAD” and “S/N RATIO.” not produce numbers that look as good as S/N RATIO. Here are six examples of sensitivity specifications as they would AUDIO DISTORTION appear in various manufacturers literature. Curiously enough, all of these measurements were made on the same receiver. Good: Less than 1% at 1KHz Excellent: Less than 0.5% at 1KHz 0.34uV input for 12dB SINAD Generally these numbers are straightforward and can be com- 0.30uV input for 12dB quieting pared directly. The distortion figures are usually taken at 1kHz. 0.27uV input for 12dB S/N This is kind of a “best case” frequency since the compandors add 0.45uV input for 20dB SINAD distortion at lower frequencies, and narrow-band IF filters can add distortion at higher frequencies. Distortion at 100Hz can be 0.47uV input for 30dB S/N 2.5% in a system that claims 0.4% at 1KHz. 1.20uV input for 50dB S/N All of these measurements can be called “sensitivity,” yet they actually measure different aspects of the receiver’s performance. Obviously it is necessary to compare “apples to apples” when making sensitivity comparisons. The above list shows how the sensitivity seems to vary depending on how the measurement is

47 Wireless Microphone Systems

DYNAMIC RANGE SPURIOUS REJECTION Good: 90dB Good: 80dB Excellent: 105dB Excellent: Greater than 100dB This number should be a straightforward measurement but some This is very similar to image rejection, but measures how well manufacturers include the limiter dynamic range and/or the gain the receiver rejects the entire range of frequencies that can be control range also. Sometimes this is done because it is cheaper applied to the receiver by any outside source. Ideally the to print better numbers than it is to design a superior product, but manufacturer will have tested the receiver from audio frequencies this can also be self-defense against someone else’s “better” to microwave frequencies. This number measures how well the numbers. Remember that the dynamic range measurement is first RF section, the IF filters and other sections reject interfering based on a minimal transmitter to receiver distance in a wireless signals. mic system. When the same measurement is made with the transmitter 50 feet or more away from the receiver, this number THIRD ORDER INTERCEPT will be significantly lower. Good: -15dBm Excellent: +1dBm or higher AM REJECTION A high third order intercept spec is a desirable receiver specifica- Good: 50dB at an unspecified RF level tion since it measures how well the receiver resists interference Excellent: 60dB over a range such as 20uV to 50mV caused by multiple interfering frequencies. Interfering frequen- This measurement shows how well the receiver rejects amplitude cies may be other wireless microphones that are being used in modulation (AM) of the RF signal caused by such things as the same location, or combinations of outside transmitters. This fluorescent lamps, bridge rectifiers in other electronic equipment, specification gives a single, excellent measure of how well the SCR light dimmers and similar power circuits. This measure- receiver resists many kinds of overload. ment, if given at all, is usually made at one RF level (the level Consider transmitters on frequencies A and B, and a receiver on that produces the best numbers of course) but should be made frequency C. If the three frequencies are equally spaced, the over a wide range since the real world is rarely so kind as to second harmonic of one of the transmitters will mix with the present an optimum RF level to the receiver. fundamental of the other transmitter, producing a signal exactly IMAGE REJECTION on the frequency of the receiver. Good: 80dB If: (A x 2) - B = C or (B x 2) - A = C Excellent: Greater than 100dB Then: the receiver will likely respond to this third order IM. In the mixer stage of all wireless receivers there are two frequen- As an example, consider transmitters on 181 MHz and 182 MHz. cies that will produce the IF frequency, and as far as the mixer is These will produce third order interference for receivers on either concerned, either frequency is equivalent. These two frequencies 183 MHz or 180 MHz. are equally spaced on either side of the oscillator frequency. For (181 x 2) - 182 = 180 instance, if the IF frequency in the receiver is 10.7 MHz and the transmitter frequency (the carrier) is 179 MHz then the local or oscillator frequency in the receiver will have to be 168.3 MHz (182 x 2) - 181 = 183 (179.0 – 168.3 = 10.7 MHz). This receiver will have an image Notice that the transmitter frequencies we chose are very close to frequency at 157.6 MHz, because the difference between 157.6 the receiver frequency. This means that the selectivity of the and the local oscillator at 168.3 is also 10.7 MHz (168.3 - 157.6 receiver is largely useless. However the better the design of the = 10.7). If it weren’t for the RF filters in the front end of the RF stage and mixer, the less of this intermodulation interference receiver, the receiver would be just as sensitive to the image will be produced. You can of course pick transmitter frequencies frequency at 157.6 MHz as the “correct” frequency of 179 MHz. that will not produce interference with a specific receiver, but this Since the image frequency of 157.6 MHz is in the taxicab service gets to be very difficult in large, multi-channel systems. Addi- band and taxicabs are allowed 75 Watts of power (wireless tionally, in some urban areas there can be hundreds of high microphones typically only 0.05 to 0.10 Watts), the receiver in powered transmitters around you, over which you have no this example must do an outstanding job of rejecting the image control. The receiver must have a high 3rd order intercept frequency. Image rejection is a function of the front-end specification. selectivity of a receiver and the IF frequency in the receiver. The LIMITER RANGE image frequency is always twice the IF frequency below the carrier for low side oscillator injection and twice above for high Good: 15dB side injection. The higher the IF frequency then the farther the Excellent: 30 dB or more image frequency is from the carrier and the easier it is to reject it This indicates the amount of audio overload the transmitter can in the front end RF filters. Better UHF receiver designs are using handle before audibly distorting the signal. A good limiter IF frequencies of 71 MHz up to 250 MHz and perhaps even allows the gain of the transmitter to be set higher, since not as higher in the future. much headroom has to be allowed to prevent audio overload. This important feature is found on only a few wireless systems, and provides an audible improvement in signal to noise ratio.

48 Interpreting Wireless Mic Specifications

BATTERY LIFE Good: 8 hours (5 hours with UHF models) alkaline 9 Volt Excellent: 12 hours (8 hours with UHF models) alkaline 9 Volt In some applications, a transmitter must operate for extended periods of 6 hours or more. If the transmitter quits before the session is complete, obviously someone is going to have a problem. In some cases, the cost of re-doing the session or performance could be significant. It is also very important that some means of evaluating the battery status be available. A “warning time” of an hour or more is generally useful. In other applications, the cost of batteries can be an important consideration. If the transmitter is used for twelve hours a week and has a battery life of 6 hours, it can amount to $250 a year, which is not a negligible sum for some budgets. (using a price of about $2.49 per battery at 12 hours per week) SPURIOUS EMISSIONS Good: 50 dB below the carrier Excellent: 60 dB below the carrier Some wireless transmitters produce frequencies other than the desired carrier. All crystal controlled transmitters start with a low frequency crystal and multiply up to the carrier output frequency. For example, starting with a 15 MHz crystal controlled oscillator, the next stage would be a tripler to 45 MHz, then a doubler stage to 90 MHz, then a doubling output stage to produce the final frequency of 180 MHz. Many low level spurious frequencies are produced in this process, but the frequencies most likely to cause problems are at the carrier frequency plus and minus the internal crystal fundamental. In the example given they would be 180 MHz plus or minus 15 MHz. Spurs would be produced by this example at 165 MHz and 195 MHz. If there were another receiver at 195 MHz in the same location, it probably would pick up the spurious frequency. TRANSMITTER OUTPUT POWER Good: 30 mW for VHF and UHF models Excellent: 50 mW for VHF; 100mW for UHF models If there is any single specification that is most abused, it is this one. 50 mW (0.05 Watts) is the maximum output power allowed by the FCC for use in VHF wireless microphones. UHF transmitters are allowed up to 250 mW, but at this power level, battery consumption becomes a factor to consider. The more power the transmitter radiates, the smaller the chances are for interference and the greater the operating range. There is a suitable "trade off," however, between output power and battery life. Some well known UHF transmitters really put out as little as 10 mW, so it is wise to look at both power output and battery life (or power consumption) when you compare specs from different manufacturers. A listing in the published specs of a particular transmitter that states that the power is less than the FCC maximum, or one that simply states the FCC allowance itself, is meaningless. All legal transmitters meet the FCC requirement, but the best performance will come from those that put out a true 50 mW in the VHF spectrum, and 100 mW (or more) in the UHF spectrum, and offer battery life long enough for the particular application.

49 Wireless Microphone Systems

50 Evaluating Wireless Microphone Systems

EVALUATING WIRELESS MICROPHONE SYSTEMS In spite of the occasional complexities of operating a wireless hand has been put between the mouth and the mic. The key test microphone system, there are some simple tests that can be will warn you to listen closely for the effect. The key test will performed (without test equipment) that can be very revealing. also reveal audio circuits that are upset by supersonics. The peak energy of jangling keys is actually around 30 kHz, well above A wireless microphone is still a “microphone” by definition. It’s human hearing. If the circuits in the transmitter don’t filter out sole purpose is to produce accurate audio for whatever applica- the supersonics, the compandor will respond grossly. This is a tion. The fact that it is “wireless” simply means that it can valid test since sibilants in the human voice also contain super- perform without an attached cable. sonics. Supersonic overload will cause sibilants to sound ragged The following tests are recommended to help you assess the as the level is driven up and down by sounds you can’t hear. quality of a particular wireless mic system before you make the decision to buy or rent the system. Each of the tests will check THE LOW FREQUENCY AUDIO ”BUMP TEST” the system for a particular type of performance and/or problem. This test will reveal the inherent signal to noise ratio of the In order to gain an overall assessment of the system quality, it is wireless system and how well the compandor handles low best to conduct as many of these tests as possible, if not all of frequency audio signals. The “inherent signal to noise ratio” is them, since you will find that some designs are very good in the signal to noise ratio before companding. some areas and poor in others. One or two tests alone is not enough to gain a good overall assessment. This test requires listening to the system in a very quiet environ- ment with minimal background noise. Place the transmitter and THE “CAR KEY TEST” microphone in a different room from the receiver, or use high isolation headphones to monitor the audio output of the receiver. This is a favorite amongst high-end wireless manufacturers. This In either case, there must be minimal background noise near the simple test reveals how well a wireless mic system can handle microphone. Background noise at a high enough level will high frequency audio transients and, in fact, the quality of the negate the test. entire audio processing chain in the system. Set up the system for normal voice levels, then place the Set up the wireless system with a pair of headphones or a sound transmitter and microphone on a table or counter. Make a fist system at a fairly high level without feedback. It is best to be with your hand and gently bump the table with the meaty part of able to listen to the audio output of the receiver away from the you hand (not your knuckle). The idea is to generate a low level, acoustic sound that the keys themselves generate. Set the input low frequency “bump” near the microphone at just enough level gain on the transmitter for a normal level with an average to open the compandor on the wireless system. speaking voice. Try varying how hard you bump the table with your fist to find a Gently shake the key ring loosely near the microphone so that the low level that just opens the compandor and listen to the results. keys jingle and rattle. Shake the keys within a foot or so of the When you“bump” the table, listen for background noise that microphone, then move them gradually away from the micro- sounds like a “whoosh” or “swish” that accompanies the sound phone while you shake them until they are as much as 8 to 10 of the bump. feet away from the mic. Listen to the audio that comes out of the receiver. Does it sound like car keys, or a bag of potato chips The idea is to listen to how much background noise is released being crushed? through the wireless system when the “bump” occurs, and also to whether or not the “bump” heard through the wireless sounds the Next, have someone talk into the wireless system while the keys same as in real life. are shaken as in the previous paragraph. Listen for distortion of the talker’s voice while the keys rattle. Move the keys from a This is an excellent test of the difference between a single-band foot or so from the microphone and then away from the micro- compandor and a dual-band compandor with DNR filtering, as phone to as much as 8 to 10 feet and listen to the effect on the well as a test of the signal to noise ratio of the wireless system. talker’s voice. With the transmitter gain set for a normal voice level during this test, the results you hear will be what the system will actually do This is a tough test for anything other than a hard-wired micro- in real use. phone. The results you hear will tell you, without argument, how well the input limiter, and compandor attack and decay times It is also interesting, although not a valid test, to set the transmit- work in the design, and give you a clear idea of the audio quality ter gain at minimum, then turn the receiver output up to maxi- you can expect from the system in real life. mum, and do the bump test again. The only reason to do this is to help understand just how much noise is actually suppressed by A loosely shaken set of metallic car keys on a key ring produces the system in normal use, and to emphasize the importance of large quantities of high frequency transients. A wireless system proper transmitter gain adjustment. that fails this test miserably, and a lot do, will also distort sibilants in the human voice. Often listeners don’t notice this A wireless mic system design that uses a large amount of pre- high frequency transient distortion because sibilants don’t have a emphasis/de-emphasis as noise reduction will likely do fairly specific frequency but are more like random noise. Distorted well in the “bump test,” however, it may also fail miserably in the random noise still sounds like noise. On a system that fails the previous “car key test.” key test, however, strong sibilants won’t have a clear, open quality but will instead have a muffled sound as if someone’s

51 Wireless Microphone Systems

CHECKING THE INPUT LIMITER RANGE on the body in the same location with the same , In this test, you will need to make some loud noises at the and apply the same criteria to define the limit of the range, or it microphone, but be able monitor the output of the receiver in a will not be a valid comparison. fairly quiet environment. It’s best done with two people. The Even if the maximum range of the system is well beyond what purpose of this test is to listen to how well the transmitter input you would normally need, this test will demonstrate the sensitiv- limiter can handle audio peaks well above the average level. ity of the receiver and how well the system handles weak signal Set up the wireless system for an average level so that the system conditions in general. indicates brief peaks at full modulation with a normal voice, with Short Range Test of Squelch and Diversity Performance: the microphone at a distance of 2 feet from the talker’s mouth. While the talker speaks at a constant level, bring the microphone A “short range” walk test checks to see how well the receiver closer and closer to their mouth. Make sure breath pops don’t handles deep multi-path nulls that occur at a close operating range with a generally strong RF signal. Do not remove the get into the microphone when it gets close to the mouth by antennas on the transmitter or receiver to worsen the conditions, keeping the microphone to the side of their mouth. If the as this will negate the validity of the test. transmitter has a poor limiter, or no limiter at all, the signal will get louder and then begin to distort as the loudness increases. In Set up the wireless system same as above, except find a location a system with a good limiter, the sound will get louder up to the where multi-path reflections will be abundant, such as an area beginning of limiting, and then will remain at a fairly level with lots of metal file cabinets or lockers, a medium to small volume even as the mic is moved closer to the mouth. metal building, a metal trailer, etc. Place the receiver antenna/s within a couple of feet or so of a metal surface to exaggerate The character of the sound may change due to the different multi-path cancellations at the antenna. The antennas on a distances as the mic is moved closer to the talker’s mouth, but the diversity receiver need to be at least a 1/2 wavelength apart to system should be able to handle a large overload without distortion. You can also test a limiter by shouting into a micro- achieve the maximum benefit of the diversity technique. If the phone, but keep in mind that the character of the talker’s will receiver cannot be configured this way in actual use, then position the antennas as they will be used. change as they go from a speaking voice to a shout. Some wireless system designs try to prevent overload by having low Walk around the area with the transmitter while speaking and try microphone gain available to the user. This compromise will to find a location where a dropout or squelch (audio mute) result in a poor signal to noise ratio when the RF signal gets occurs. Moving the transmitter around within a couple feet of a weak. A sharp audio peak produced by hand claps or other metal surface may help to generate a multi-path condition. means is also a good test of the limiter action. The idea in this test is to see how prone the system is to produc- THE ”WALK TESTS” ing dropouts, and to look for loud noise bursts that occur during a dropout if and when one does occur. An effective diversity As the name implies, this is a test where one person takes a walk system will make it difficult to find a dropout, which will tell you while talking into the transmitter, and the other person listens to something about the effectiveness of the diversity circuitry. If the receiver output. and when a dropout does occur with a strong average RF level at There are two different “walk tests” for a wireless system. the receiver, the receiver should simply mute the audio during the ¨ Check the maximum operating range dropout and not allow any noise or noise burst to occur. ¨ Check the short range squelch and diversity performance An aggressive squelch system in the receiver is best in a close Before conducting either of these tests, the wireless mic system range situation, as it will eliminate noise bursts created by should be set up exactly the way it will be used. The microphone dropouts, however, it will also limit the maximum operating and transmitter must be in the exact postition on the talker’s body range as in the previous test. A less aggressive squelch allows where they will be used, and the receiver must be connected to maximum operating range, but will generally allow noise bursts whatever equipment it will feed, with power and antennas to occur during dropouts at close range. connected and positioned as in actual use. Unless the wireless These two tests illustrate the dilemma of a conventional squelch system is set up this way, the results of the walk tests will not be system in having to choose between either close range or distant realistic. Do not remove antennas on the transmitter or receiver operating range, and also illustrates the benefit of an adaptive to try to simulate extreme operating range, as this will alter the squelch system like the Lectrosonics SmartSquelchTM which way some receivers work, such as Lectrosonics models that use automatically configures itself for close range or long distance TM TM SmartSquelch and SmartDiversity circuitry. operation as the system is being used. The tests are also a good Checking for Maximum Range: proving ground for Lectrosonics SmartDiversityTM. The classic walk test is to see how far away you can get with the After conducting both types of walk tests, you will have a good transmitter before dropouts are bad enough to make the system idea of what to expect in actual use. Some systems may provide unusable. You can walk until a count of 8 to 10 dropouts occur, excellent maximum range characteristics, but prove to be noisy for example, and define that as the limit of the range. Or, walk in short range, multi-path conditions. Other systems may be until the dropouts or hiss buildup is objectionable according to great at the short range test, but be poor performers in the your own assessment. When comparing two or more different maximum range test. Of course, the ideal wireless system would wireless systems, it is very important to repeat the same exact do well in both tests. path for each walk test, position the receivers and the transmitters

52 Evaluating Wireless Microphone Systems

THE “HARD-WIRED A-B TEST” This is a matter of setting up two identical microphones, one connected with an audio cable and the other with a wireless system, to perform a listening test. The trick to this is to get the listening levels of both setups to be exactly the same. Even a very slight difference in level will “fool” the ears into hearing differences that may not actually exist. Place the microphones equidistant from a sound source or someone’s mouth so the the same audio signal enters both microphones. Switch back and forth between the cabled and the wireless setup as the listener compares the sound of each setup. This, of course, is best done in a “blindfold” manner where the listener has no way of telling which setup is being monitored, and by writing down a few notes about the results. As a “reality check,” it is always good idea to swap the micro- phones and listen to them a second time to see if there are slight differences in the microphones themselves that may have been detected in the first comparison.

53 Wireless Microphone Systems

54 Glossary

GLOSSARY OF WIRELESS MICROPHONE TERMS

AM: Amplitude Modulated (carrier level shifts) INTERMODULATION: Also referred to as “IM.” The mixing of two or more signals, producing sums, differences and har- CARRIER: The operating frequency of a wireless system. A monic multiples. IM generally occurs in the gain amplifier ahead fixed frequency radio signal which is shifted up and down of the mixer stage within a receiver, but also occurs in any non- (modulated) in either frequency (FM) or level (AM) by the audio linear device. signal. ISOTROPIC RADIATOR: A completely non-directional COMPANDOR: A noise reduction circuit which employs an antenna (one which radiates equally well in all directions.) This encode/decode process. The transmitter encodes (compresses) antenna exists only as a mathmatical concept and is used as a the dynamics of the audio signal and the receiver decodes theoretical reference to measure . (expands) the dynamics of the audio signal. The compressor in the transmitter and the expander in the receiver must be perfectly LOW SIDE INJECTION: A superhet receiver design in which complementary. the oscillator frequency is below the carrier frequency. dBi: Decibels of gain compared to an isotropic radiator. MIXER: The circuit or component in a superhet receiver where the oscillator signal is combined with the incoming carrier signal. dBd: Decibels of gain compared to a dipole antenna MULTI-PATH: The presence of multiple signals arriving at the NOTE: dBd is 3dB above dBi receiver antenna simultaneously. Signals that are in phase will add to one another. Signals that are out of phase will cancel one DETECTOR (DEMODULATOR): The circuit in a receiver another. which is used to recover the intelligence (audio) from a signal. OSCILLATOR: An electronic circuit which generates a signal DIVERSITY: A method of reducing or eliminating multi-path at a specific frequency. dropouts by using two or more antennas and/or receivers. The most popular methods include dual-antenna phase switching, RECEIVER IMAGE: A second frequency that a superhet dual-receiver audio switching and “ratio diversity” audio receiver will respond to. The image frequency is two times the combining. The most effective method is ratio diversity combin- IF frequency either above or below the carrier frequency, ing. depending upon whether the receiver design is “low side” or “high side” injection. An RF signal on the “image” frequency of DROP OUT: A momentary loss of the carrier and sound, or a the receiver will produce a difference signal in the mixer just as buildup of background noise when the transmitter is in a certain valid as the intended IF signal created by mixing the oscillator location in the room. Moving the transmitter (even a few inches) with the carrier. usually restores the sound to normal. RECEIVER: The device that picks up the radio signal from the FM: Frequency Modulated (carrier frequency shifts) transmitter, converts it into an audio signal and feeds audio into your sound system or recorder. FRONT-END: The first stage of filtering in a receiver. The first circuit stage following the antenna input to the receiver. RF NOISE: Radio signals generated by something other than the transmitter. Usually sounds like hiss, static or hash. RFI HIGH SIDE INJECTION: A superhet receiver design in (Radio Frequency Interference) may be AM or FM, but the effect which the oscillator frequency is above the carrier frequency. is that it either alters the audio signal, or adds background noise to the audio signal. IF: Intermediate Frequency. Refers to the resulting signal in a superhet receiver after the incoming carrier is mixed with the RF: Radio Frequency. Also used generally to refer to the radio oscillator signal. signal generated by the system transmitter, or to energy present from other sources that may be picked up by a wireless receiver. IM PERFORMANCE: A measure of the ability of the receiver to reject signals which are capable of producing IM products. RFI: Radio Frequency Interference. A non-desired radio signal which creates noise or dropouts in the wireless system or noise in IMAGE REJECTION: A measure of the ability of the receiver a sound system. RFI can be generated by a wide variety of to reject RF signals present on the image frequency of the sources including electronic organs, computers, switching power receiver created by the mixer. Image rejection is one of the supplies, broadcast radio signals and outside radio devices. purposes of front-end filtering in a superhet receiver. Radio signal energy can enter a sound system component or alter the audio signals in cabling, producing annoying hiss, whining or intelligible audio signals. Proper shielding and balanced audio cabling are the best defense against RFI problems in a sound system. High quality receivers are the best defense against RFI in wireless microphone systems.

55 Wireless Microphone Systems

SELECTIVITY: The ability of a receiver to reject interfering signals close to the desired carrier frequency.

SENSITIVITY: The ability of a receiver to operate on very weak RF signal levels.

SQUELCH: A muted, or silent, condition in the receiver. When the radio signal from a transmitter is too weak to produce a quality audio signal, the receiver will shut off or “squelch.” “Squelch” is also used to refer to the circuit in the receiver that provides the audio muting.

SUPERHET: Short for “super-heterodyne.” The mixing of two signals producing a third signal. Wireless microphone receivers (and almost all other receivers) utilize an oscillator, producing a signal which is mixed with the incoming radio signal from the receiver antenna to produce a lower frequency signal (the IF signal).

SUPERSONIC NOISE SQUELCH: A fairly popular method of muting the audio output of a receiver when the supersonic noise reaches a preset level. The assumption is that noise buildup above the audio passband (20 to 30KHz range) is an indication that the signal to noise ratio of the system is inadequate to produce a usable audio signal.

THIRD ORDER INTERCEPT: A measure of how well the receiver resists interference caused by multiple interfering signals. This specification gives a single, excellent measure of how well the receiver resists many kinds of overload. It is directly related to the RF compression level.

TRANSMITTER: The device worn (or held) by the user which sends or “transmits” the sound from the microphone to the receiver. The transmitter actually converts the electrical signal coming from the microphone into a radio signal and then “transmits” it out through some sort of antenna.

UHF: (generally 300MHz to 3000MHz)

VHF: (30 to 300MHz) High Band wireless systems are usually 150MHz to 216MHz Low band wireless systems are usually 30MHz to 50MHz

56 Wireless Microphone Applications

WIRELESS MICROPHONE APPLICATIONS FREE-LANCE SOUND AND VIDEO PRODUCERS used with a wide variety of lavalier, hand-held and shotgun There are thousands of “free-lance” video producers across the microphones, and feature a convenient input coupler that is easy US. Most of these producers are individuals who contract with to use, but provides a secure attachment for the microphone. teleproduction companies, networks, ad agencies or corporate As cameras become smaller, the size of a receiver that can be clients for video and audio production services. Most free-lance mounted directly on a camera must also be small. Modern video producers are geared for field production, with limited post cameras (especially digital models) radiate RF noise, which can production services. also limit the choices of receivers that can be mounted on the A unique type of multi-channel wireless audio system has camera. Only receivers with very high selectivity can reject the emerged in free-lance use that is often called a “bag system.” RFI from the camera and retain usable operating range. Once This is a portable rig in an over-shoulder carrying case that again, receiver selectivity is king. contains several wireless receivers, a portable mixer, one or two WORSHIP CENTERS wireless transmitters connected to the output of the mixer, a boom pole with mic and cabling, a variety of audio adapters, With well over 450,000 worship centers in the USA alone, this is walkie-talkies, a tape recorder and sometimes even spare perhaps the largest market for wireless microphone systems. batteries for the videographer. It’s an awfully busy little kit, and While the types of services provided by various worship houses a tough environment for wireless. vary substantially, the way they use wireless mic systems is fairly consistent. Applications in worship centers can require lavalier The audio output of the mixer is fed to one or two transmitters and/or hand-held transmitters to make various parts of the that send the signal to the camera receiver/s so that the whole worship services more effective. video/audio rig is completely portable. This is the technique used for the vast majority of television documentary production Worship centers are “predictable” users, since worship services are in the field. normally offered on a regularly scheduled basis in the same location. Weddings, funeral services and other types of gatherings The primary challenge for wireless operation in a bag system like can occur at varying times, but most of the usage is scheduled in this rests in the fact that the mixer output transmitters are often advance. Some worship centers are concerned about the cost of within inches of the receivers in the bag. It is never a good idea batteries used to operate the wireless transmitter, while others are to place transmitters and receivers this close together in any more concerned that the batteries may fail during use, causing a multi-channel wireless system, however, there is no other choice disruption of the service. In either case, however, the usage is still to attain this level of portability for field production. The check- predictable and planned in advance, in contrast to some applica- out procedure outlined in the INTERFERENCE section of this tions, such as news gathering, where the wireless usage is with guide is mandatory to ensure reliable operation. little or no notice and almost always in a new location. Since the whole purpose of a bag system is portability, it goes The acoustic environment in most worship centers is usually very without saying that the system will constantly be moved from quiet, with a closely attentive listener, and generally with a good one location to another, often from city to city. Thus, interfer- quality sound system. This makes even subtle problems in the ence from television broadcast signals and a myriad of other wireless system very obvious to the listeners. All of this de- sources cannot be predicted. This means that the only way to get mands that the wireless system provide a very high signal to a rig like this to work reliably is to use very high quality receivers noise ratio, excellent audio quality and a strong resistance to with excellent selectivity and IM performance, and make a habit interference and dropout problems. of checking out the system thoroughly with each new location. It’s not easy, but this is how it’s done. In some types of services, the RF environment can become more difficult if the services include extensive use of electronic musical BROADCAST NEWS GATHERING (ENG) instruments. Strong RF energy can be radiated from many types of The rough and tumble, mad scramble of gathering the news is no electronic synthesizers, organs, and other electronic instruments, place for a wimpy wireless. The receivers must be compact and generating enough RF noise to cause problems with dropouts and portable for mounting on video cameras, and the transmitters operating range in the wireless mic systems. A walk-test of the must be rugged enough to withstand the physical abuse that is the wireless system with everything turned on should always be very nature of this environment. The RF link has to work on the performed any time electronic musical instruments are to be used first try, since there is no time to re-assemble everyone and re- with the wireless, especially when new electronics have been shoot the scene. This might explain why very few companies added to the sound system. make wireless systems that provide reliable operation for ENG. Budget and cost considerations are always a concern for worship Most ENG crews use only one or two wireless systems at any centers. Finding the right balance between cost and the neces- given moment, which eases the complexities of operating the sary performance is not difficult to determine, it just takes a little wireless systems, somewhat. There is, however, rarely time to go thought in the beginning. It is always good practice to install the through any check-out procedures, so only the most selective, highest quality wireless mic system the budget will allow, even if high performance receivers will work reliably. it means waiting a bit longer for the purchase. Plug-on transmitters are the primary choice of professional ENG crews, due to the versatility they provide to meet varying microphone requirements for each shot. The best designs can be

57 Wireless Microphone Systems

MOTION PICTURE PRODUCTION THEATRE Motion picture productions represent one of the most presti- The performing arts includes several applications for wireless gious, yet demanding applications for wireless microphone microphone systems. A prime example is theatre, which is systems. The techniques used for recording audio for film have normally a multi-channel wireless application. A stage production also changed from earlier years. Dialogue is now recorded live in is rehearsed in advance, which helps check out the wireless the scene, “sweetened” or modified in post-production and then system, but it is actually performed live in front of an audience, so synchronized with the picture in the composite mix. In years reliable performance of the wireless system is often more critical past, the final audio was often overdubbed in post-production, than in a recorded event. Some theatre applications are virtually using the audio recorded on the set as only a reference for re- permanent installations where the production always occurs in the recording. same facility. Other theatre shows travel around the country, which With the advent of high quality wireless microphone systems, the makes operating a multi-channel wireless system far more need for over-dubbing the actors’ dialogue in post production has complicated. Some of the larger traveling theatre shows employ largely disappeared. The audio captured during the filming is full time staff to set up and operate the wireless microphone used for the final track, placing an acute demand on the wireless systems. systems to deliver outstanding audio quality. The most effective and practical wireless receiver systems for The demand for very high quality wireless mic systems in the film theatre are rack mounted assemblies, using antenna and power business is due in part to the excessive cost of re-shooting a scene. distribution systems. Frequency coordination is a must in any A single scene lasting only a few seconds can easily cost thousands multi-channel application, but it is an on-going battle in a of dollars to produce. If the wireless mic system drops outs and traveling show. Intermodulation between the wireless systems the scene has to be re-shot, or the audio has to be overdubbed in themselves is always a concern, but usually it can be avoided or post-production, it can be quite expensive. Another reason for the minimized through proper frequency coordination and careful demand for high quality, high reliability wireless mic systems is installation. Interference from external sources, however, due to the fact that modern theaters are equipped with high quality becomes almost inevitable if the show is traveling. The only type sound systems. Most people also now own or often listen to high of wireless systems that should be used for theatre are very high quality sound systems in their homes, often with a digital signal performance systems using high selectivity receivers. source. The average consumer or movie goer has been trained to SOUND STAGE expect higher quality sound. “Sound reinforcement companies,” or what might be called With the advances in signal processing over the last few years, “touring companies,” are prominent wireless microphone users in there is a growing desire in many post-production engineers to the public eye, even though they represent a relatively small utilize close miking techniques in the field, and then modify the market in terms of the number of wireless systems used. Large audio later to make it “fit” the picture. The sound of a person’s touring companies are highly visible due to the fact that they often voice will vary greatly depending upon how much ambient noise, produce the audio for nationally televised performances and major echo and reverberation is included in the mix. For example, it is musical groups. The applications for wireless systems in this easy to tell if the sound of someone’s voice is in a large room market generally range from hand-held transmitters for singing, to (like a warehouse) or a small room by simply listening to the belt-pack models for musical instruments. amount of background noise and echoes. It is easy to make a person’s voice sound like it is outdoors, in a large room, etc. Touring companies that work on a national or international basis through equalization and by adding additional background noise, face difficult frequency coordination problems as they travel echoes and reverberation, but it is very difficult, if not impos- across the country or around the world. Touring companies that sible, to remove background noise and echoes if they are work on a local or regional basis may not face as many problems contained in the original recording. Wireless mic systems are with frequency coordination or reliability, but the sound quality is used extensively in motion picture production since they make still just as important as in a major telecast or concert. close miking techniques easy to accomplish. In some cases, the On a sound stage, the receiver antenna/s can usually be placed location sound mixer may include background noise in the scene close to the transmitters. This helps to overcome the problem of to make the sound “fit” the picture better. dropouts, and can even help to reduce interference from external Wireless mic systems must interface with many types of other sources in the vicinity, but the problems generated by RF noise devices on a typical film production set. For example, fishpoles sources on stage (digital instruments and signal processing) can and “plant” mics are being used with wireless transmitters, which still create major problems. takes less time to set up than the cabled equivalents. Production All major productions involve detailed frequency coordination sound mixers require a variety of different types of microphones and testing before the production begins. The venue where the for various applications and voice types, and often have personal production occurs will usually have more radio devices in use preferences with regard to which microphone capsules are best. than just the wireless mics used on stage. There will invariably Thus, it is imperative that a wireless system for film production be communications radios and often a wireless intercom system be capable of working with all popular types of microphone in use at a major production. The operating frequencies and capsules. Accurate level metering on the transmitter also comes intermodulation of all the radio devices must be taken into in very handy when the receiver is not within sight. account before there can be any reasonable assurance that all the systems will operate reliably. High selectivity receivers are a must in this type of application.

58 Wireless Microphone Applications

Most sound reinforcement applications consist of a “musical speakers and so on. In many cases, a sound company will format.” With respect to musical formats, there are several basic contract to travel with the evangelist on a larger venue just as types of venues: they would with a band. Concerts A traveling evangelical production is generally the same as a At a live concert, the production is with a large audience, and is concert with respect to the equipment used to stage the produc- often televised as well. It is critical that the wireless systems be tion. The sound system is normally a high output PA system, the as reliable as possible since there is no chance to go back and do production is staged live, with many of them televised live or a particular scene over again. The sound system is operated recorded for later broadcast. The requirements for wireless manually, so the squelch capabilities of the receivers are usually microphone systems in this application are the same as any large not critical, since the channel is manually muted on the console scale concert, and the RF environment is every bit as difficult. when a particular wireless system is not in use. Most concerts CORPORATE APPLICATIONS involve multi-channel wireless systems, which means that the receiver must exhibit outstanding selectivity and IM perfor- Corporate television production mance. Specialized antennas, multi-couplers and other equip- Many medium to larger sized companies have departments ment for multi-channel systems will commonly be used. In whose sole function is to produce video tapes for either advertis- addition, computer interfaces for the wireless systems can be ing or training. The cost of producing the video tapes internally very helpful to bring remote control and monitoring to a conve- versus the cost of hiring them done outside of the company, and nient location, which is often away from the receiver rack. the decreasing cost of quality video equipment, has given rise to The receivers feed sound into very high output PA systems, many programs being produced internally. The corporate user making the signal to noise ratio of the system a critical design may either rent or purchase wireless systems, depending upon consideration. Since the systems are normally used for singing how often they need to use them. and musical instruments, the input overload capabilities of the The corporate video user utilizes wireless microphone systems to transmitter are also very important. produce recorded materials. This lessens the importance of the One of the most problematic situations that occurs with touring wireless system reliability, since a scene could oftentimes be shot companies producing concerts is the matter of frequency again if there were problems, but the inconvenience of having to selection and coordination. Since the concert is almost always a reshoot a scene still outweighs a slightly higher cost of a high “one shot deal,” there is rarely an opportunity to re-work the quality wireless. frequency coordination plan after a live concert has started. Corporate Trainers Intermodulation is highly complex and it is absurd to try to There has always been a need for wireless microphone systems switch frequencies on a problem system after the program has for use by traveling “trainers” in the corporate market. These are started. By switching frequencies on one system to solve an RF either company employees or hired professional speakers who problem, several new problems could easily be generated on travel across the country or around the world giving lectures or other wireless systems. Frequency switchable receivers and conducting training seminars to groups as small as 20 or 30, or to transmitters offer little or no help once the show has started. groups as large as several hundred. The use of a wireless system, The frequency of one wireless system cannot be changed without which enables them to also use other visual aids, has become a considering the implications it could have on other systems. standard requirement. Frequency changes can only be made between shows when there A wireless system for a traveling “trainer” must operate in a wide is time to also work on the overall frequency coordination plan. variety of geographic locations without interference. The A major touring company will normally have “spare” wireless receiver in the system must usually provide enough audio outputs systems on coordinated frequencies ready and waiting if a to accommodate both sound system feeds and recorder feeds. problem occurs. Since the trainers must travel, size and weight are also a major Night clubs factor in choosing an appropriate system to meet their needs. It While sound quality is no less critical in a night club than at a is often convenient for the receiver to be battery powered, so that concert, night clubs can have a distinct advantage. If the contract the presenter can locate the receiver near the audio input jacks to is for the same band and sound company to produce a number of the house sound system without having to run long power cords, similar shows on the same stage over a period of several days or or carry long audio cables with them as they travel. The same, weeks, wireless problems can be resolved with predictable compact receivers commonly used in field production prove to results, since the location often remains unchanged between be very useful for travelling presenters and public speakers. shows. WEDDING AND EVENT VIDEOGRAPHERS Traveling Evangelists Videotaping weddings has become as common as photographing Another application for wireless systems in sound reinforcement weddings. While it would seem logical for photographers to begin is with traveling evangelists. The venue in this application to include video production in weddings, this is generally not the usually involves lavalier systems for speaking, hand-held systems case. This means that at a wedding there will often be both a for singing, and belt-pack systems for musical instruments. video company and a photographer working. The video produc- Multi-channel wireless systems with as many as 10 channels are tion must also include audio which can create some annoyances often used. The stage will often include a variety of musical for the photographer, such as microphone cables, clip-on mics, etc. instruments, such as synthesizers and MIDI controlled keyboards A wireless microphone system not only improves the audio and percussion sets, electric guitars, power amplifiers, monitor quality, but it also makes it easier for the video crew and photogra-

59 Wireless Microphone Systems pher to work together (no cabling on the floor, and so on). A belt- Magicians make up a small, but interesting market for wireless pack transmitter that works with almost any type of mic will microphone systems. In many cases, a magician will conceal a permit the use of special lavalier models designed to be concealed. microphone inside a variety of different costumes, which means Most entry level wedding videographers begin offering their that they require a versatile transmitter that will adapt to a wide services with consumer level video products. Most of this is due variety of microphones. In most cases, magicians travel to to the cost of the equipment, and the fact that many wedding different events, so the physical size and RF performance of the videographers only shoot weddings on a part-time basis, while receiver is a concern. Most magicians prefer to “travel light,” so keeping full-time jobs elsewhere. In addition, the cost of a very a compact receiver would be preferred. Interference is always a high quality video production of a wedding (multiple cameras concern for travelers, so the selectivity and IM suppression and separate sound mixer) makes it impractical for most wedding performance of the receiver are important considerations. budgets. The videographer can rarely charge enough for their Bingo Halls are usually multi-million dollar operations with services to permit the use of professional and broadcast level budgets that can easily afford the convenience of wireless. Bingo equipment. This presents a curious paradox. On one hand, a Halls use wireless microphones for the staff members who walk wedding application dictates a very high quality wireless system, the floors to verify the bingo cards at the end of a game. Generally since they can’t go back and do it over again. One the other speaking, a bingo hall needs multiple transmitters, but never needs hand, however, it is difficult to justify the cost of broadcast to run more than one of them at a time. The most cost-effective quality equipment because of the limited budgets typical of wireless systems for them include a single receiver with multiple weddings. “push to talk” transmitters. Audio lost in the production due to a wireless problem at a wedding cannot be re-caputured conveniently. Even though, in theory, the exchanging of wedding vows could be re-enacted, it would certainly not gain any points for the producer with the bride, the groom or either of their families. This points to the importance of prior planning and the use of high quality wireless systems. Since most weddings occur in worship centers, it is also very important to coordinate the wireless systems used for the video production with those used in the worship center. This includes a thorough walk-test and check out before the ceremony starts, with all of the wireless systems in place and turned on as they will actually be used during the ceremony. In a multiple camera production, it makes sense to use a single transmitter with a matching receiver on each camera. In more advanced productions, multiple wireless mic systems will be placed appropriately, with the matching receivers in a central location feeding an audio mixer. The output of the mixer can then be fed to one or more wireless transmitters to radio the mixed audio back to the camera or cameras. This sort of setup, of course, requires the same advance planning and check out as any multi-channel wireless system. A recorder can also be used at the mixer location to capture a redundant audio track that can be useful if a problem occurs with the wireless feed to the cameras during the cermony. SPECIALIZED APPLICATIONS Other applications for wireless microphone systems include: Auctioneers use wireless microphone systems extensively, since they require the mobility that wireless affords. Most of the wireless systems sold into the auctioneering market are part of complete portable sound systems, normally used with type microphones for hands-free use. Band Directors use wireless microphone systems for marching band rehearsals and on the field during performances. The wireless systems used for rehearsals are normally combined with some type of portable sound system. The wireless transmitters used during performances can range from belt-pack transmitters mounted on instruments to hand-held transmitters that feed receivers connected directly into the house or stadium sound system.

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